archiver: Fix test case for srpm archiver mode

We can't assume that rpm packaging will always be enabled when running
oe-selftest. In particular when using nodistro instead of poky this is
not enabled by default.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>