[BEAM-6263] Fix error-prone test setup for FlinkJobServerDriver

[mxm]

Please see the four commits which address different issues with the test setup:

• [BEAM-6263] Prevent NullPointer on concurrent JobServer shutdown
• [BEAM-6263] Prevent port collisions in FlinkJobServerDriveTest
• [BEAM-6263] Prevent stderr->stdout redirection
• [BEAM-6263] Restore stderr on exceptions to print the error

Post-Commit Tests Status (on master branch)

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

[mxm]

Unrelated test failure: 20:00:05 Execution failed for task ':beam-sdks-java-io-hadoop-format:test'.

[mxm]

Filed https://issues.apache.org/jira/browse/BEAM-6268 Alexey is working on a fix.

All other tests pass. Will merge because this fixes crucial testing issues which even led the error messages being hidden from the test results.