[SPARK-20456][PYTHON][FOLLOWUP] Fix timezone-dependent doctests in unix_timestamp and from_unixtime
[SPARK-20456][PYTHON][FOLLOWUP] Fix timezone-dependent doctests in unix_timestamp and from_unixtime

## What changes were proposed in this pull request?

This PR proposes to simply ignore the results in examples that are timezone-dependent in `unix_timestamp` and `from_unixtime`.

```
Failed example:
    time_df.select(unix_timestamp('dt', 'yyyy-MM-dd').alias('unix_time')).collect()
Expected:
    [Row(unix_time=1428476400)]
Got:unix_timestamp
    [Row(unix_time=1428418800)]
```

```
Failed example:
    time_df.select(from_unixtime('unix_time').alias('ts')).collect()
Expected:
    [Row(ts=u'2015-04-08 00:00:00')]
Got:
    [Row(ts=u'2015-04-08 16:00:00')]
```

## How was this patch tested?

Manually tested and `./run-tests --modules pyspark-sql`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18597 from HyukjinKwon/SPARK-20456.

someorz/spark,eyalfa/spark,shuangshuangwang/spark,ConeyLiu/spark,ahnqirage/spark,aray/spark,aray/spark,ueshin/apache-spark,zhouyejoe/spark,ueshin/apache-spark,aosagie/spark,debugger87/spark,caneGuy/spark,debugger87/spark,akopich/spark,witgo/spark,lxsmnv/spark,jiangxb1987/spark,goldmedal/spark,sahilTakiar/spark,1haodian/spark,aosagie/spark,rezasafi/spark,brad-kaiser/spark,guoxiaolongzte/spark,lxsmnv/spark,zuotingbing/spark,rekhajoshm/spark,szhem/spark,ddna1021/spark,michalsenkyr/spark,vinodkc/spark,shubhamchopra/spark,witgo/spark,rezasafi/spark,shubhamchopra/spark,shubhamchopra/spark,stanzhai/spark,spark-test/spark,ddna1021/spark,aray/spark,tejasapatil/spark,shubhamchopra/spark,Aegeaner/spark,shaneknapp/spark,bdrillard/spark,esi-mineset/spark,darionyaphet/spark,taroplus/spark,michalsenkyr/spark,nchammas/spark,dongjoon-hyun/spark,bravo-zhang/spark,1haodian/spark,mahak/spark,jkbradley/spark,WindCanDie/spark,skonto/spark,ajaysaini725/spark,sureshthalamati/spark,eyalfa/spark,jiangxb1987/spark,yanboliang/spark,skonto/spark,alunarbeach/spark,WeichenXu123/spark,lvdongr/spark,debugger87/spark,xflin/spark,xuanyuanking/spark,zhouyejoe/spark,WeichenXu123/spark,someorz/spark,vinodkc/spark,rednaxelafx/apache-spark,actuaryzhang/spark,alunarbeach/spark,maropu/spark,shaneknapp/spark,someorz/spark,nlalevee/spark,milliman/spark,darionyaphet/spark,zuotingbing/spark,xuanyuanking/spark,sahilTakiar/spark,szhem/spark,LantaoJin/spark,ericvandenbergfb/spark,aosagie/spark,wangmiao1981/spark,yanboliang/spark,ericvandenbergfb/spark,shaneknapp/spark,holdenk/spark,wangyum/spark,gengliangwang/spark,LantaoJin/spark,HyukjinKwon/spark,caneGuy/spark,holdenk/spark,joseph-torres/spark,ericvandenbergfb/spark,stanzhai/spark,kiszk/spark,zhouyejoe/spark,wzhfy/spark,actuaryzhang/spark,yanboliang/spark,zzcclp/spark,ahnqirage/spark,pgandhi999/spark,stanzhai/spark,yanboliang/spark,michalsenkyr/spark,apache/spark,BryanCutler/spark,zzcclp/spark,ioana-delaney/spark,icexelloss/spark,actuaryzhang/spark,esi-mineset/spark,cin/spark,brad-kaiser/spark,saltstar/spark,wangmiao1981/spark,taroplus/spark,gengliangwang/spark,adrian-ionescu/apache-spark,narahari92/spark,1haodian/spark,darionyaphet/spark,chuckchen/spark,skonto/spark,wzhfy/spark,alunarbeach/spark,facaiy/spark,hhbyyh/spark,facaiy/spark,zzcclp/spark,minixalpha/spark,srowen/spark,janewangfb/spark,adrian-ionescu/apache-spark,dbtsai/spark,mdespriee/spark,jkbradley/spark,SHASHANKB/spark,highfei2011/spark,ueshin/apache-spark,someorz/spark,chuckchen/spark,ajaysaini725/spark,chuckchen/spark,HyukjinKwon/spark,minixalpha/spark,mdespriee/spark,caneGuy/spark,ConeyLiu/spark,yanboliang/spark,nilsgrabbert/spark,srowen/spark,ptkool/spark,kevinyu98/spark,nilsgrabbert/spark,shubhamchopra/spark,shaneknapp/spark,milliman/spark,ueshin/apache-spark,ron8hu/spark,WindCanDie/spark,vinodkc/spark,akopich/spark,esi-mineset/spark,stanzhai/spark,byakuinss/spark,kiszk/spark,chuckchen/spark,zero323/spark,someorz/spark,ahnqirage/spark,JoshRosen/spark,akopich/spark,wzhfy/spark,SHASHANKB/spark,kevinyu98/spark,cin/spark,witgo/spark,esi-mineset/spark,rikima/spark,zuotingbing/spark,xflin/spark,mike0sv/spark,xflin/spark,maropu/spark,spark-test/spark,techaddict/spark,ron8hu/spark,ptkool/spark,srowen/spark,cloud-fan/spark,shuangshuangwang/spark,chuckchen/spark,lvdongr/spark,rekhajoshm/spark,rekhajoshm/spark,narahari92/spark,darionyaphet/spark,SHASHANKB/spark,markhamstra/spark,sahilTakiar/spark,mahak/spark,adrian-ionescu/apache-spark,techaddict/spark,darionyaphet/spark,mike0sv/spark,bdrillard/spark,facaiy/spark,wzhfy/spark,nchammas/spark,adrian-ionescu/apache-spark,ueshin/apache-spark,joseph-torres/spark,adrian-ionescu/apache-spark,lxsmnv/spark,stanzhai/spark,cin/spark,ajaysaini725/spark,yanboliang/spark,hhbyyh/spark,HyukjinKwon/spark,sahilTakiar/spark,akopich/spark,janewangfb/spark,rezasafi/spark,highfei2011/spark,zuotingbing/spark,UndeadBaneGitHub/spark,alunarbeach/spark,zero323/spark,JoshRosen/spark,mdespriee/spark,caneGuy/spark,janewangfb/spark,Aegeaner/spark,dbtsai/spark,ddna1021/spark,kevinyu98/spark,wangyum/spark,ptkool/spark,nilsgrabbert/spark,szhem/spark,nilsgrabbert/spark,tejasapatil/spark,nchammas/spark,sureshthalamati/spark,icexelloss/spark,chuckchen/spark,caneGuy/spark,caneGuy/spark,rednaxelafx/apache-spark,dbtsai/spark,pgandhi999/spark,brad-kaiser/spark,pgandhi999/spark,kevinyu98/spark,cloud-fan/spark,BryanCutler/spark,mike0sv/spark,kiszk/spark,maropu/spark,zero323/spark,bravo-zhang/spark,1haodian/spark,gengliangwang/spark,mahak/spark,skonto/spark,WeichenXu123/spark,tejasapatil/spark,ron8hu/spark,ptkool/spark,ptkool/spark,1haodian/spark,highfei2011/spark,goldmedal/spark,eyalfa/spark,ajaysaini725/spark,bravo-zhang/spark,narahari92/spark,akopich/spark,rikima/spark,eyalfa/spark,saltstar/spark,actuaryzhang/spark,gengliangwang/spark,zhouyejoe/spark,apache/spark,mike0sv/spark,saltstar/spark,mdespriee/spark,zhouyejoe/spark,actuaryzhang/spark,zhouyejoe/spark,ptkool/spark,akopich/spark,maropu/spark,janewangfb/spark,spark-test/spark,witgo/sp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