[FLINK-9949][tests] Kill Flink processes in DB/teardown!

[GJL]

What is the purpose of the change

Not killing Flink processes at the end of a test, can cause interference with subsequent test runs.

Brief change log

• Kill Flink processes in DB/teardown!.

Verifying this change

This change added tests and can be verified as follows:

• Ran tests in docker.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

cc: @tillrohrmann

[tillrohrmann]

LGTM. Merging this PR.