test/lib/mutation_source_test: add consistent log to all methods
test/lib/mutation_source_test: add consistent log to all methods

Most test methods log their own name either via testlog.info() or
BOOST_TEST_MESSAGE() so failures can be more easily located. Not all do
however. This commit fixes this and also converts all those using
BOOST_TEST_MESSAGE() for this to testlog.info(), for consistency.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla