ccanlint: make fewer tests compulsory.

Compulsory means "malformed", we might get rid of it altogether, since
any test can mark "fail" and make ccanlint exit with non-zero status.

Now we only have four compulsory tests:
   info_exists               Module has _info file
   depends_exist             Module's CCAN dependencies can be found
   objects_build             Module object files can be built
   module_builds             Module can be built from object files