packit: run tests on Fedora Rawhide via Testing Farm

Let's utilize the full power of Packit and run some tests with the just
built RPMs. This makes use of the Fedora infrastructure provided by
the Testing Farm project [0][1].

With the current configuration, the `tests` job runs tests from the
Fedora tests repository [2] in a very similar fashion like Ubuntu CI
does, just with different metadata all around it. ATTOW there are only
two tests, which are wrappers around unit tests and integration tests;
the latter one currently runs only nspawn-based tests, since there's no
KVM on the test VMs, and, for now, I'd like to see how well the infra is
going to manage our upstream traffic and how stable the whole thing is
end up being before increasing the work load.

[0] https://docs.testing-farm.io/Testing%20Farm/0.1/index.html
[1] https://packit.dev/docs/configuration/upstream/tests
[2] https://src.fedoraproject.org/tests/systemd

.packit.yml

@@ -44,3 +44,11 @@ jobs:
   - fedora-rawhide-ppc64le
   - fedora-rawhide-s390x
   - fedora-rawhide-x86_64
+
+- job: tests
+  trigger: pull_request
+  fmf_url: https://src.fedoraproject.org/tests/systemd
+  fmf_ref: main
+  tmt_plan: ci
+  targets:
+  - fedora-rawhide-x86_64