Add CI job with unstable netmgr unit tests
Some tests of the netmgr unit test tend to fail in some environments but not in others. Running it in a well-defined environment should help with making those tests stable.
I did not leverage gcc:softhsm2.6
as it's a bit "special".
Validation pipeline: https://gitlab.isc.org/isc-projects/bind9/-/pipelines/67438.
Closes #2599 (closed)
Edited by Michal Nowak