### What changes were proposed in this pull request?
This PR changes the test to get an active executorId and set it as preferred location instead of setting a fixed preferred location.
### Why are the changes needed?
The test is flaky. After checking the [log](https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/124086/artifact/core/), I find the root cause is:
Two test cases from different test suites got submitted at the same time because of concurrent execution. In this particular case, the two test cases (from DistributedSuite and BarrierTaskContextSuite) both launch under local-cluster mode. The two applications are submitted at the SAME time so they have the same applications(app-20200615210132-0000). Thus, when the cluster of BarrierTaskContextSuite is launching executors, it failed to create the directory for the executor 0, because the path (/home/jenkins/workspace/work/app-app-20200615210132-0000/0) has been used by the cluster of DistributedSuite. Therefore, it has to launch executor 1 and 2 instead, that lead to non of the tasks can get preferred locality thus they got scheduled together and lead to the test failure.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
The test can not be reproduced locally. We can only know it's been fixed when it's no longer flaky on Jenkins.
Closes#28849 from Ngone51/fix-spark-32000.
Authored-by: yi.wu <yi.wu@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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