[LIVY-647][TEST]Fix travis failed on batch session should not be gc-ed until application is finished

[runzhiwang]

What changes were proposed in this pull request?

Fix travis failed on "batch session should not be gc-ed until application is finished"

The cause of failed is as follows:

1. When create BatchSessionManager, the GarbageCollector thread will be created, which collect garbage according to session.state. However the session was mocked, and the test thread execute doReturn(s).when(session).state several times, if the test thread do half of the statement doReturn(s).when(session).state, and GarbageCollector thread check session.state at this time, the exception will threw by GarbageCollector thread.

2. So the fix avoid executing doReturn(s).when(session).state after session has been registered into SessionManager.

How was this patch tested?

Existed UT and IT.

[codecov-io]

Codecov Report

Merging #222 into master will increase coverage by 0.41%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #222      +/-   ##
============================================
+ Coverage     68.34%   68.75%   +0.41%     
- Complexity      919      923       +4     
============================================
  Files           100      100              
  Lines          5715     5742      +27     
  Branches        867      884      +17     
============================================
+ Hits           3906     3948      +42     
+ Misses         1242     1234       -8     
+ Partials        567      560       -7

Impacted Files	Coverage Δ	Complexity Δ
...main/scala/org/apache/livy/server/LivyServer.scala	35.96% <0%> (+0.49%)	11% <0%> (ø)	⬇️
...ain/java/org/apache/livy/rsc/driver/RSCDriver.java	80.08% <0%> (+2.11%)	42% <0%> (+1%)	⬆️
...c/main/scala/org/apache/livy/repl/ReplDriver.scala	33.33% <0%> (+2.56%)	7% <0%> (ø)	⬇️
.../scala/org/apache/livy/sessions/SessionState.scala	61.11% <0%> (+2.77%)	2% <0%> (ø)	⬇️
...cala/org/apache/livy/scalaapi/ScalaJobHandle.scala	55.88% <0%> (+2.94%)	7% <0%> (ø)	⬇️
...apache/livy/server/batch/BatchSessionServlet.scala	95.45% <0%> (+4.27%)	3% <0%> (ø)	⬇️
...ain/scala/org/apache/livy/utils/SparkYarnApp.scala	70.58% <0%> (+5.22%)	40% <0%> (ø)	⬇️
...in/java/org/apache/livy/rsc/driver/JobWrapper.java	88.57% <0%> (+5.71%)	9% <0%> (+1%)	⬆️

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[jerryshao]

Run the test again.

[jerryshao]

LGTM. Merging to master branch.