tests: mutation_source_tests: Always print the seed
tests: mutation_source_tests: Always print the seed

BOOST_TEST_MESSAGE() is not logged by default, and for some tests we
don't want to enable that because it's too noisy. But we need to know
the seed to reproduce a failure, so we better to always print it.

avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla