CI: Run c9s+c10s mock builds, to verify %check passes there

For example, due to old versions of setuptools or tox,
the expected results of some of the tests might need adjustments.

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 c10s/c9s backports.

plan.fmf

@@ -4,6 +4,12 @@ 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: c10s_mockbuild
+      path: /
+      test: NAME="CentOS Stream" VERSION_ID=10 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: