[SPARK-32645][INFRA] Upload unit-tests.log as an artifact
[SPARK-32645][INFRA] Upload unit-tests.log as an artifact

### What changes were proposed in this pull request?

This PR proposes to upload `target/unit-tests.log` into the artifact so it will be able to download here:
![Screen Shot 2020-08-18 at 2 23 18 PM](https://user-images.githubusercontent.com/6477701/90474095-789e3b80-e15f-11ea-87f8-e7da3df3c03e.png)

### Why are the changes needed?

Jenkins has this feature. It should be best to have the same dev functionalities with it.
Also, note that this was pointed out https://github.com/apache/spark/pull/29225#discussion_r471485011.

### Does this PR introduce _any_ user-facing change?

No, dev-only

### How was this patch tested?

https://github.com/apache/spark/actions/runs/213000777 should demonstrate it

Closes #29454 from HyukjinKwon/SPARK-32645.

Authored-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
Signed-off-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

JoshRosen/spark,shaneknapp/spark,ueshin/apache-spark,shaneknapp/spark,shuangshuangwang/spark,ptkool/spark,milliman/spark,shuangshuangwang/spark,wzhfy/spark,JoshRosen/spark,mahak/spark,jiangxb1987/spark,JoshRosen/spark,hvanhovell/spark,xuanyuanking/spark,wzhfy/spark,holdenk/spark,zero323/spark,chuckchen/spark,witgo/spark,gengliangwang/spark,milliman/spark,holdenk/spark,maropu/spark,WeichenXu123/spark,wangyum/spark,JoshRosen/spark,nchammas/spark,mahak/spark,nchammas/spark,zero323/spark,vinodkc/spark,WeichenXu123/spark,nchammas/spark,jiangxb1987/spark,xuanyuanking/spark,rednaxelafx/apache-spark,srowen/spark,BryanCutler/spark,mahak/spark,wzhfy/spark,xuanyuanking/spark,shuangshuangwang/spark,milliman/spark,holdenk/spark,zero323/spark,cloud-fan/spark,hvanhovell/spark,jiangxb1987/spark,chuckchen/spark,ueshin/apache-spark,wangmiao1981/spark,wzhfy/spark,srowen/spark,zzcclp/spark,nchammas/spark,apache/spark,mahak/spark,hvanhovell/spark,zero323/spark,hvanhovell/spark,WeichenXu123/spark,taroplus/spark,nchammas/spark,maropu/spark,HyukjinKwon/spark,jiangxb1987/spark,cloud-fan/spark,ptkool/spark,mahak/spark,dongjoon-hyun/spark,witgo/spark,srowen/spark,wangyum/spark,zero323/spark,chuckchen/spark,dongjoon-hyun/spark,milliman/spark,zero323/spark,nchammas/spark,wangyum/spark,xuanyuanking/spark,vinodkc/spark,ptkool/spark,gengliangwang/spark,WeichenXu123/spark,ueshin/apache-spark,shaneknapp/spark,maropu/spark,srowen/spark,taroplus/spark,HyukjinKwon/spark,maropu/spark,cloud-fan/spark,gengliangwang/spark,taroplus/spark,HyukjinKwon/spark,shuangshuangwang/spark,HyukjinKwon/spark,wangyum/spark,wangmiao1981/spark,wzhfy/spark,srowen/spark,zero323/spark,dongjoon-hyun/spark,maropu/spark,zzcclp/spark,WeichenXu123/spark,wangyum/spark,dongjoon-hyun/spark,milliman/spark,JoshRosen/spark,zzcclp/spark,vinodkc/spark,vinodkc/spark,ptkool/spark,gengliangwang/spark,ptkool/spark,ueshin/apache-spark,vinodkc/spark,rednaxelafx/apache-spark,HyukjinKwon/spark,rednaxelafx/apache-spark,apache/spark,vinodkc/spark,taroplus/spark,cloud-fan/spark,srowen/spark,gengliangwang/spark,apache/spark,zzcclp/spark,milliman/spark,wangyum/spark,taroplus/spark,vinodkc/spark,witgo/spark,holdenk/spark,witgo/spark,wangmiao1981/spark,apache/spark,WeichenXu123/spark,chuckchen/spark,HyukjinKwon/spark,cloud-fan/spark,hvanhovell/spark,dongjoon-hyun/spark,shuangshuangwang/spark,wangyum/spark,taroplus/spark,cloud-fan/spark,milliman/spark,ptkool/spark,xuanyuanking/spark,taroplus/spark,jiangxb1987/spark,nchammas/spark,dongjoon-hyun/spark,witgo/spark,gengliangwang/spark,BryanCutler/spark,rednaxelafx/apache-spark,jiangxb1987/spark,mahak/spark,shuangshuangwang/spark,wangmiao1981/spark,dongjoon-hyun/spark,JoshRosen/spark,zzcclp/spark,WeichenXu123/spark,BryanCutler/spark,hvanhovell/spark,hvanhovell/spark,maropu/spark,HyukjinKwon/spark,apache/spark,xuanyuanking/spark,wzhfy/spark,ueshin/apache-spark,wzhfy/spark,zzcclp/spark,rednaxelafx/apache-spark,BryanCutler/spark,JoshRosen/spark,apache/spark,holdenk/spark,apache/spark,witgo/spark,holdenk/spark,shaneknapp/spark,maropu/spark,chuckchen/spark,gengliangwang/spark,chuckchen/spark,shuangshuangwang/spark,srowen/spark,rednaxelafx/apache-spark,witgo/spark,wangmiao1981/spark,BryanCutler/spark,BryanCutler/spark,ptkool/spark,ueshin/apache-spark,shaneknapp/spark,wangmiao1981/spark,shaneknapp/spark,cloud-fan/spark,xuanyuanking/spark,ueshin/apache-spark,rednaxelafx/apache-spark,mahak/spark,shaneknapp/spark,zzcclp/spark,chuckchen/spark,BryanCutler/spark,holdenk/spark,jiangxb1987/spark,wangmiao1981/spark