[BEAM-5913] Re-enable parallel builds for Java Jenkins jobs

[kennknowles]

This re-enables parallel builds for Java jobs where it was disabled.

Recent context:

• The root cause of parallel build failures was identified and fixed in [BEAM-5887] Fix classifier for unshaded tests jars #6864
• Parallel tests were re-enabled, but exposed an existing bug in Dataflow Worker tests: https://issues.apache.org/jira/browse/BEAM-5908
• The bug was reproduced without parallel builds in DO NOT MERGE: Exhibit race condition obviously #6878
• The bug was fixed in [BEAM-5907] Add CountDownlatch to racy Dataflow Worker test #6877

We should be able to re-enable them, since we know we have fixed the one failure that occurred last time.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

[kennknowles]

R: @swegner

You noticed quick last time, perhaps have good context this time around. What do you think? Enough assurance to run the seed job on this PR and try it out?

[swegner]

Yes, we should run the seed job and kick off some parallel tests.

[swegner]

Run Seed Job

[kennknowles]

run java precommit

[kennknowles]

run java postcommit

[kennknowles]

Run Java PortabilityApi PostCommit

[kennknowles]

Failure in ElasticSearchIO in precommit. Will re-run to see if it is a general flake or a flake-made-permanent by parallel builds.
https://builds.apache.org/job/beam_PreCommit_Java_Phrase/353/
https://scans.gradle.com/s/j3fvhwf6br3oi

[kennknowles]

run java precommit

[kennknowles]

In super cool news, it took 17 minutes instead of 55.

[kennknowles]

Looks like they all passed.

[kennknowles]

Postcommits were 2.5 hrs instead of 3.5 so there's still some work to be done there.

[swegner]

Awesome! LGTM

[kennknowles]

This was not to be rebased and squashed, please be careful.