common: unit tests for config::expand_meta

common: unit tests for config::expand_meta

Part of the config.cc tests are in test/confutils.cc but they do not
cover meta variable expansion. Create unittest_config for config.{h,cc}
specific tests.

The test_md_config_t is made a friend of md_config_t to allow testing
private and protected methods.

test/cli/ceph-conf/show-config-value.t is used to check that the human
readable message message shows as expected when there is an expansion
loop.

Signed-off-by: Loic Dachary <loic@dachary.org>