test: Clean /usr/local less aggressively in TestMenu.testBasic

Only remove the cockpit/systemd directory that we added, not the whole
parent.

test/verify/check-shell-menu

@@ -68,7 +68,7 @@ class TestMenu(testlib.MachineCase):
         m.execute("mkdir -p /usr/local/share/cockpit/systemd; cp -rp /usr/share/cockpit/systemd/* /usr/local/share/cockpit/systemd")
         m.execute(
             """sed -i '/"menu"/a "memory": { "label": "Memory", "path": "#/memory" },' /usr/local/share/cockpit/systemd/manifest.json""")
-        self.addCleanup(m.execute, "rm -r /usr/local/share/cockpit")
+        self.addCleanup(m.execute, "rm -r /usr/local/share/cockpit/systemd")
 
         self.login_and_go("/system")