common: fix large output in unittest_daemon_config

All tests in daemon_config use the global g_ceph_context
object. Introducing an expansion loop in it will impact all tests and
generate a very large output.

Remove the SubstitutionLoop test case which is also covered in
test/common/test_config.cc.

Signed-off-by: Loic Dachary <loic@dachary.org>