[9.20] fix: ci: Ensure that junit.xml is present and non-empty after each system...

Previously, JUnit files were not generated or were generated empty for various reasons for some system/unit test runs.

Now, the number of tests collected for a MR is up from about 4k to 5.8k in the "Tests" tab of a pipeline.

Additionally, there is a check that ensures that [a somewhat sane](https://gitlab.isc.org/isc-projects/bind9/-/commit/c5a271eb8beb9912501ec564de3bb669ba02507d) `junit.xml` file is generated after every system/unit test job and fails the job otherwise.

Closes #5316

Backport of MR !10556

Merge branch 'backport-5316-ensure-junit-xml-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10649