dnf: add dummy packages for testing

dnf: add dummy packages for testing

oeqa/runtime needs packages which are always built, but we don't want to
use packages which have large side effects (unlike the current test recipe,
run-postinsts).

As finding a package that is both of these things isn't easy, for now
dnf can generate its own.  Moving forward this should be generalised and
all of the package manager tests unified.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>