test: rewrite test-exec-deserialization.py

test: rewrite test-exec-deserialization.py

Rewrite the test in bash and make it part of our integration test suite,
so it's actually executed in all our upstream CI environments.

The original test is flaky in environments where daemon-reload might
occur during the test runtime (e.g. when running the test in parallel
with the systemd-networkd test suite). Also, it was run only in CentOS
CI in limited way (i.e. without sanitizers), since it tests the host's
systemd, instead of the just built one.

Resolves: #29943