The ability to conveniently mark tests which should only be run when the
CI_ENABLE_ALL_TESTS environment variable is set seems to be useful on a
general level and therefore it should not be limited to the "timeouts"
system test, where it is currently used.
pytest documentation [1] suggests to reuse commonly used test markers by
putting them all in a single Python module which then has to be imported
by test files that want to use the markers defined therein. Follow that
advice by creating a new bin/tests/system/pytest_custom_markers.py
Python module containing the relevant marker definitions.
Note that "import pytest_custom_markers" works from a test-specific
subdirectory because pytest modifies sys.path so that it contains the
paths to all parent directories containing a conftest.py file (and
bin/tests/system/ is one). PyLint does not like that, though, so add a
relevant PyLint suppression.
The above changes make bin/tests/system/timeouts/conftest.py redundant,
so remove it.
[1] https://docs.pytest.org/en/7.0.x/how-to/skipping.html#id1
