Tests: enable TF tag repository during the unit-tests plan

This should solve https://issues.redhat.com/browse/TFT-2810 and make
sure that the packages that the tested RPM was built against, are
available on the VM for installation.

Related: RHEL-56049

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

plans/unit-tests.fmf

@@ -1,10 +1,13 @@
 summary: Run osbuild unit tests from dist-git sources
 prepare:
-    how: install
+    - how: install
-    package:
+      package:
-        - git
+          - git
-        - make
+          - make
-        - osbuild
+          - osbuild
+    - how: shell
+      order: 20
+      script:  dnf config-manager --set-enabled testing-farm-tag-repository
 discover:
     how: fmf
     filter: tag:unit-test