tests: Add check-qobject for equality tests

Add a new test file (check-qobject.c) for unit tests that concern
QObjects as a whole.

Its only purpose for now is to test the qobject_is_equal() function.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171114180128.17076-7-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
