oeqa/selftest: Ensure packages classes are set correctly for maintainers test
The dnf packages aren't parsed if rpm isn't in PACKAGE_CLASSES which means
the aintainers test failes for OE-Core (where ipk is the default) but not
for poky (where the default is rpm).
Ensure PACKAGE_CLASSES is set so it works in all cases.
[YOCTO #14277]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit
842b11107363357ed933cfcf619f1cf23f0d841e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
