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 (the subset including
stratis), and skips all unrelated ones. So runs will be faster, and are
also less prone to failures unrelated to Stratis.

[1] cockpit-project/cockpit@18814b60c97cbbba

(cherry picked from commit 03123c3)

plans/cockpit.fmf

@@ -14,7 +14,7 @@ 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