Fix flaky GitSCMTest#testSubmoduleFixup

[Vlatombe]

The test should wait for the build to complete before actually checking
its status.

java.lang.AssertionError:
unexpected build status; build log was:
------
Started by upstream project "test0" build number 1
originally caused by:
 Legacy code started this job.  No cause information is available
Running as SYSTEM
Building in workspace /jenkins/workspace/builder/output-git/work/git/target/tmp/j h69583182000798596/workspace/test1
The recommended git tool is: NONE
No credentials specified
Cloning the remote Git repository
Cloning repository /jenkins/workspace/builder/output-git/work/git/target/tmp/junit3569465322501079491/junit1849114342070567213
 > git init /jenkins/workspace/builder/output-git/work/git/target/tmp/j h69583182000798596/workspace/test1 # timeout=10

------

Expected: is <SUCCESS>
     but: was null
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
	at org.junit.Assert.assertThat(Assert.java:964)
	at org.jvnet.hudson.test.JenkinsRule.assertBuildStatus(JenkinsRule.java:1439)
	at org.jvnet.hudson.test.JenkinsRule.assertBuildStatusSuccess(JenkinsRule.java:1468)
	at hudson.plugins.git.GitSCMTest.testSubmoduleFixup(GitSCMTest.java:1675)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
	at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:54)
	at org.jvnet.hudson.test.JenkinsRule$1.evaluate(JenkinsRule.java:601)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:299)
	at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:293)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.lang.Thread.run(Thread.java:748)

Fix flaky GitSCMTest#testSubmoduleFixup

Wait for downstream build completion before checking its status.

Checklist

• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
• No spotbugs warnings were introduced with my changes
• Documentation in README has been updated as necessary
• Online help has been added and reviewed for any new or modified fields

Types of changes

• Test

[MarkEWaite]

Thanks very much for detecting that mistake in the test.

[MarkEWaite]

Thanks for the fix!

These tests intentionally use the if construct rather than the Assume construct because the Assume construct generates a warning about skipped tests that then is shown as a warning in the warnings ng plugin output. I'd prefer to continue using the early exit without using a JUnit construct so that the warning is not generated.

Alternately, if the waitForCompletion resolves the flakiness in the test, then let's remove the early return and let the test run on Windows.

[MarkEWaite]

I've started a test series on my infra to see if the Assume can be removed for Windows. Will report the results here.

[MarkEWaite]

Proposing two changes to check the behavior on ci.jenkins.io