CI: Run c9s mock build with EPEL enabled, to verify %check passes there

Note that CentOS Stream mock builds run with EPEL (Next) enabled,
hence we also enable tox tests in %check for better coverage.
Hence, running this makes sense even for actual c9s backports.

(cherry picked from Fedora commit b95c0052f0)

plan.fmf

@@ -4,6 +4,9 @@ discover:
   - name: same_repo
     how: shell
     tests:
+    - name: c9s_mockbuild
+      path: /
+      test: NAME="CentOS Stream" VERSION_ID=9 tests/mocktest.sh pyproject-rpm-macros
     - name: pytest
       path: /tests
       test: ./mocktest.sh python-pytest

pyproject-rpm-macros.spec

@@ -5,8 +5,8 @@ License:        MIT
 
 %bcond tests 1
 # pytest-xdist and tox are not desired in RHEL
-%bcond pytest_xdist %{undefined rhel}
-%bcond tox_tests %{undefined rhel}
+%bcond pytest_xdist %[%{undefined rhel} || %{defined epel}]
+%bcond tox_tests %[%{undefined rhel} || %{defined epel}]
 
 # The idea is to follow the spirit of semver
 # Given version X.Y.Z: