This allows the list of directories which contain MIME data to be set,
separately from the list of directories returned by
g_get_user_data_home() and g_get_system_data_dirs().

While the latter are overridden for a unit test, we don’t have access to
the system MIME registry, which can sometimes be useful for tests which
need to know about standard MIME associations from shared-mime-info.

Allow g_content_type_set_mime_dirs() to be used from unit tests to allow
them to use the system MIME registry again, or to allow them to be
pointed to another custom registry.

Signed-off-by: Philip Withnall <withnall@endlessm.com>