oeqa/runtime/systemd.py: skip instead of failing without avahi
The SystemdServiceTests assume that avahi-daemon is installed,
which is not necessarily the case depending on the image being
tested.
Better check this dependency before starting the tests and skip
them if the service is not installed. This has to be done for
each test instead of for the entire module, because other
tests in the module can run without avavi.
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
