Decrease Log4j testing time

[ppkarwasz]

Once tests are stable (#1776), we can improve the testing time.

A first step will consist in:

• running all tests in parallel in separate JVMs; the status quo is running them in separate JVMs one at a time,
• improve the most "sleepy" tests, by replacing arbitrary sleep() calls.

[ppkarwasz]

The tests are running in parallel now: https://github.com/apache/logging-log4j2/actions/runs/6244539913

[ppkarwasz]

Many of our tests were written copying other tests, which also means, they write to the same files, they use the same ports, etc. Therefore to achieve this goal it was necessary to decouple many tests:

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