Clean shared state pollution to avoid flaky tests in testCombiner

What is the purpose of this change

This PR is to fix 2 non-idempotent flaky tests:

org.apache.hadoop.mapred.TestOldCombinerGrouping.testCombiner org.apache.hadoop.mapreduce.TestNewCombinerGrouping.testCombiner

Why the tests failed

For test testcombiner, it writes to the directory TEST_ROOT_DIR whose path is determined by UUID. When running this test twice, the TEST_ROOT_DIR created by the first running already exists, so the second running cannot create the directory with the same name, which will lead to a RuntimeException. This will pollute the shared state. It may be better to clean state pollutions so that some other tests won't fail in the future due to the shared state polluted by this test.

Reproduce test failure

Run the test twice in the same JVM.

Expected result

The tests should run successfully without any failure when multiple tests that use this state are run in the same JVM.

Actual result

Failure from test run:

java.lang.RuntimeException: Could not create test dir: /home/...

Fix

Fully delete the directory created for this test when it ends.

Link to PR: