ci: Adjust Cockpit test plans

Cockpit recently reorganized its fmf test plans to better balance the
test durations [1]. Follow suit.

It would be really nice to have teemtee/tmt#1770
to avoid this duplication..

As a bonus, this now *only* runs storage tests, and skip all unrelated
ones. So runs will be faster, and are also less prone to failures
unrelated to Stratis.

cockpit-project/cockpit@18814b60c97cbbba

plans/cockpit.fmf

@@ -14,7 +14,12 @@ discover:
 execute:
     how: tmt
 
-/optional:
-    summary: Run tests for optional packages (including storage)
+/storage-basic:
+    summary: Basic storage tests
     discover+:
-        test: /test/browser/optional
+        test: /test/browser/storage-basic
+
+/storage-extra:
+    summary: More expensive storage tests (LVM, LUKS, Anaconda)
+    discover+:
+        test: /test/browser/storage-extra