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 842b1110)
Signed-off-by: Steve Sakoman <steve@sakoman.com>