testmap: Run cockpit scenario in centos-7 refreshes

The only scenario where centos-7 gets provisioned in cockpit is
"expensive" (by definition).

This makes tracking flakes by context more regular, as flakes in this
test run would previously appear as a "fedora-N" context in image
refreshes, but as "fedora-N/expensive" in cockpit PRs.

lib/testmap.py

@@ -240,7 +240,7 @@ def contexts(image, *scenarios: Iterable[str]):
 IMAGE_REFRESH_TRIGGERS = {
     # some tests run against centos-7's cockpit-ws for backwards compat testing
     "centos-7": [
-        TEST_OS_DEFAULT + "@cockpit-project/cockpit",
+        f"{TEST_OS_DEFAULT}/expensive@cockpit-project/cockpit",
     ],
     "openshift": [
         "rhel-7-9@cockpit-project/cockpit/rhel-7.9",