[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 <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

.github/workflows/build_and_test.yml

@@ -183,6 +183,12 @@ jobs:
       with:
         name: test-results-${{ matrix.modules }}-${{ matrix.comment }}-${{ matrix.java }}-${{ matrix.hadoop }}-${{ matrix.hive }}
         path: "**/target/test-reports/*.xml"
+    - name: Upload unit tests log files
+      if: failure()
+      uses: actions/upload-artifact@v2
+      with:
+        name: unit-tests-log-${{ matrix.modules }}-${{ matrix.comment }}-${{ matrix.java }}-${{ matrix.hadoop }}-${{ matrix.hive }}
+        path: "**/target/unit-tests.log"
 
   # Static analysis, and documentation build
   lint: