Stabilize Log4j tests

[ppkarwasz]

Currently most of our build runs succeed only after a couple of reruns (cf. Github Actions).

This is an umbrella issue to have a reliable CI and consists of:

• fixing the longstanding Flaky tests on the CI #1568,
• fix the contention for OS resources (files and TCP/IP ports) between tests, that prevents us from using even the most basic form of parallelization.

[ppkarwasz]

This task also requires improvements in the way we log test failure, by:

• improving how the per-test ListStatusListener works: 16cfa41, ba8e21d, fb683e1, d564b6e
• adding a ListErrorHandler with similar semantics as ListStatusListener: 565f57d

[ppkarwasz]

In order to prevent multiple tests from using the same configuration files I introduced a default configuration file location in f68d7ca. By default you can add configuration files in the same package as the test class with .java replaced by .xml.