[SPARK-23847][FOLLOWUP][PYTHON][SQL] Actually test [desc|acs]_nulls_[first|last] functions in PySpark
[SPARK-23847][FOLLOWUP][PYTHON][SQL] Actually test [desc|acs]_nulls_[first|last] functions in PySpark

## What changes were proposed in this pull request?

There was a mistake in `tests.py` missing `assertEquals`.

## How was this patch tested?

Fixed tests.

Author: hyukjinkwon <gurwls223@apache.org>

Closes #21035 from HyukjinKwon/SPARK-23847.

facaiy/spark,techaddict/spark,rezasafi/spark,HyukjinKwon/spark,lxsmnv/spark,lxsmnv/spark,jkbradley/spark,sahilTakiar/spark,cloud-fan/spark,jiangxb1987/spark,icexelloss/spark,shaneknapp/spark,pgandhi999/spark,kiszk/spark,zhouyejoe/spark,dongjoon-hyun/spark,michalsenkyr/spark,shuangshuangwang/spark,markhamstra/spark,ueshin/apache-spark,srowen/spark,spark-test/spark,srowen/spark,darionyaphet/spark,nchammas/spark,spark-test/spark,HyukjinKwon/spark,zuotingbing/spark,HyukjinKwon/spark,xuanyuanking/spark,hvanhovell/spark,cloud-fan/spark,zuotingbing/spark,holdenk/spark,milliman/spark,ueshin/apache-spark,rednaxelafx/apache-spark,ConeyLiu/spark,szhem/spark,holdenk/spark,aosagie/spark,techaddict/spark,tengpeng/spark,rezasafi/spark,lvdongr/spark,gengliangwang/spark,ConeyLiu/spark,yanboliang/spark,zero323/spark,nchammas/spark,goldmedal/spark,mahak/spark,markhamstra/spark,jiangxb1987/spark,xuanyuanking/spark,zuotingbing/spark,spark-test/spark,highfei2011/spark,ptkool/spark,rikima/spark,hvanhovell/spark,szhem/spark,michalsenkyr/spark,eyalfa/spark,rekhajoshm/spark,witgo/spark,xuanyuanking/spark,dongjoon-hyun/spark,rekhajoshm/spark,holdenk/spark,pgandhi999/spark,bdrillard/spark,wangyum/spark,jkbradley/spark,rezasafi/spark,srowen/spark,taroplus/spark,lvdongr/spark,tengpeng/spark,WindCanDie/spark,zero323/spark,wzhfy/spark,dbtsai/spark,WindCanDie/spark,maropu/spark,ddna1021/spark,witgo/spark,rezasafi/spark,LantaoJin/spark,debugger87/spark,apache/spark,bravo-zhang/spark,skonto/spark,BryanCutler/spark,icexelloss/spark,jiangxb1987/spark,witgo/spark,szhem/spark,ConeyLiu/spark,ahnqirage/spark,milliman/spark,wangyum/spark,bravo-zhang/spark,HyukjinKwon/spark,maropu/spark,zhouyejoe/spark,shaneknapp/spark,mahak/spark,caneGuy/spark,chuckchen/spark,eyalfa/spark,yanboliang/spark,xuanyuanking/spark,skonto/spark,Aegeaner/spark,vinodkc/spark,ptkool/spark,milliman/spark,maropu/spark,zero323/spark,bravo-zhang/spark,WeichenXu123/spark,markhamstra/spark,tejasapatil/spark,mahak/spark,joseph-torres/spark,aosagie/spark,mdespriee/spark,zhouyejoe/spark,aosagie/spark,wangmiao1981/spark,ddna1021/spark,yanboliang/spark,hhbyyh/spark,WeichenXu123/spark,joseph-torres/spark,lxsmnv/spark,srowen/spark,kevinyu98/spark,spark-test/spark,wangyum/spark,wangmiao1981/spark,zzcclp/spark,lxsmnv/spark,bdrillard/spark,cloud-fan/spark,holdenk/spark,shaneknapp/spark,wangmiao1981/spark,HyukjinKwon/spark,JoshRosen/spark,xuanyuanking/spark,mahak/spark,darionyaphet/spark,zero323/spark,wangyum/spark,wzhfy/spark,ptkool/spark,milliman/spark,guoxiaolongzte/spark,rezasafi/spark,BryanCutler/spark,bravo-zhang/spark,actuaryzhang/spark,skonto/spark,facaiy/spark,apache/spark,xuanyuanking/spark,hhbyyh/spark,mahak/spark,rednaxelafx/apache-spark,darionyaphet/spark,Aegeaner/spark,dbtsai/spark,nchammas/spark,debugger87/spark,vinodkc/spark,aosagie/spark,lvdongr/spark,chuckchen/spark,rednaxelafx/apache-spark,apache/spark,rikima/spark,ddna1021/spark,guoxiaolongzte/spark,taroplus/spark,rekhajoshm/spark,jiangxb1987/spark,kevinyu98/spark,tejasapatil/spark,rekhajoshm/spark,maropu/spark,ConeyLiu/spark,bravo-zhang/spark,pgandhi999/spark,actuaryzhang/spark,wzhfy/spark,shuangshuangwang/spark,darionyaphet/spark,mahak/spark,Aegeaner/spark,mahak/spark,joseph-torres/spark,debugger87/spark,WindCanDie/spark,kiszk/spark,rekhajoshm/spark,shaneknapp/spark,LantaoJin/spark,wangmiao1981/spark,michalsenkyr/spark,mdespriee/spark,ddna1021/spark,dbtsai/spark,rikima/spark,tengpeng/spark,skonto/spark,ptkool/spark,taroplus/spark,goldmedal/spark,apache/spark,ptkool/spark,jkbradley/spark,shaneknapp/spark,hvanhovell/spark,LantaoJin/spark,highfei2011/spark,sahilTakiar/spark,jiangxb1987/spark,HyukjinKwon/spark,rikima/spark,ddna1021/spark,vinodkc/spark,gengliangwang/spark,wangmiao1981/spark,spark-test/spark,darionyaphet/spark,zzcclp/spark,debugger87/spark,joseph-torres/spark,techaddict/spark,rednaxelafx/apache-spark,WeichenXu123/spark,gengliangwang/spark,actuaryzhang/spark,BryanCutler/spark,tejasapatil/spark,szhem/spark,caneGuy/spark,taroplus/spark,michalsenkyr/spark,chuckchen/spark,icexelloss/spark,bdrillard/spark,lxsmnv/spark,JoshRosen/spark,zhouyejoe/spark,actuaryzhang/spark,szhem/spark,taroplus/spark,zzcclp/spark,ueshin/apache-spark,cloud-fan/spark,ueshin/apache-spark,tejasapatil/spark,yanboliang/spark,zzcclp/spark,maropu/spark,markhamstra/spark,vinodkc/spark,milliman/spark,milliman/spark,LantaoJin/spark,kevinyu98/spark,BryanCutler/spark,hhbyyh/spark,WindCanDie/spark,gengliangwang/spark,guoxiaolongzte/spark,actuaryzhang/spark,Aegeaner/spark,highfei2011/spark,spark-test/spark,darionyaphet/spark,holdenk/spark,guoxiaolongzte/spark,JoshRosen/spark,guoxiaolongzte/spark,taroplus/spark,facaiy/spark,kiszk/spark,shaneknapp/spark,matthewfranglen/spark,mdespriee/spark,highfei2011/spark,zzcclp/spark,jkbradley/spark,zhouyejoe/spark,lxsmnv/spark,Aegeaner/spark,ahnqirage/spark,gengliangwang/spark,bdrillard/spark,WindCanDie/spark,eyalfa/spark,caneGuy/spark,tengpeng/spark,michalsenkyr/spark,yanboliang/spark,rekhajoshm/spark,ahnqirage/spar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