oe-selftest: distrodata: change test_maintainers() to use tinfoil

oe-selftest: distrodata: change test_maintainers() to use tinfoil

Use tinfoil to enumerate recipes and get the value of RECIPE_MAINTAINER
to make it a bit more reliable in the face of do_checkpkg issues we are
currently seeing on the Yocto Project autobuilder. This also makes it a
little less painful to re-execute test_maintainers() since you don't
have to wait for bitbake -c checkpkg to complete every time.

Note that the new test has been written in such a way that it will still
function if RECIPE_MAINTAINER values are ever moved to the recipes.
Also, the test still currently fails as there are recipes that don't
have an assigned maintainer.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>