testing-farm: enable mock-stable copr repo

This is the place where the "currently being released RPMs related to
Mock" (e.g. distribution-gpg-keys) are built.  At least needed for the
cases when the RPMs are not yet in composes.

mock/integration-tests/setup-playbook/tasks/main.yml

@@ -1,4 +1,8 @@
 ---
+- name: install also RPMs from the stable copr repository
+  community.general.copr:
+    name: '@mock/mock-stable'
+
 - name: install mock and other needed packages
   package: name={{ packages }} state=latest
   vars: