oe-selftest: add a test for recipes without maintainers
'bitbake -c checkpkg world' is moved to class initializer to avoid
it being run twice in a row.
The no-maintainers test checks only oe-core recipes, as other layers
may be be configured, and assigning maintainership to specific people via
maintainers.inc is known to be used only in oe-core.
Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
