Fix Test Flakiness

[Alisha-0321]

Description:

This test is flakily fails. I run this test many times and it makes assertion fails. The failure message is as follows.

Failure:
Running com.alibaba.wasp.executor.TestExecutorService
Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.249 sec <<< FAILURE!
testExecutorService(com.alibaba.wasp.executor.TestExecutorService) Time elapsed: 0.166 sec <<< FAILURE!
java.lang.AssertionError: expected:<10> but was:<5>
at org.junit.Assert.fail(Assert.java:93)
at org.junit.Assert.failNotEquals(Assert.java:647)
at org.junit.Assert.assertEquals(Assert.java:128)
at org.junit.Assert.assertEquals(Assert.java:472)
at org.junit.Assert.assertEquals(Assert.java:456)
at com.alibaba.wasp.executor.TestExecutorService.testExecutorService(TestExecutorService.java:109)

Results :
Failed tests:
TestExecutorService.testExecutorService:109 expected:<10> but was:<5>

Tests run: 1, Failures: 1, Errors: 0, Skipped: 0

What changes were proposed in this pull request?

I observe a previous suggested fix injected some delay, but I believe that fix might be unstable in a CI environment or when run on different machines, given the dependency on some constant wait time. I suggest a new way to fix the test by adding some synchronization for the test execution only. I at first identify the code location whose slow execution leads to the flaky test failure, I found com/alibaba/wasp/executor/EventHandler.java#193 slows the execution makes assertion failure. Hence, I introduce one variable in this test class that is only there to provide some synchronization. Basically, until these statements are executed, I force the thread that runs the test to wait before it accesses the assertion values. The waiting location is at com/alibaba/wasp/executor/TestExecutorService.java#82.

Solution: I run the test many times and it makes the test pass always.

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