fix: test: Various system test stability fixes for CI

Doubling system-test parallelism and removing the `@pytest.mark.flaky` markers exposed a set of timing-sensitive failures across CI. This branch keeps only the fixes for failures that are test-robustness bugs. Other - frequent - failures often guarded by `@pytest.mark.flaky` have their separate MRs already.

Merge branch 'mnowak/system-test-stability-fixes' into 'main'

See merge request isc-projects/bind9!12171