test: Re-relax the {become,drop}_superuser() checks

The stricter checks introduced in commit 0b21500 don't yet work on
older cockpit Shell versions on debian-stable and ubuntu-2204. There the
class is still called `.pf-c-modal-box__title`, and so the selectors
don't match.

This also isn't important for this helper function. Leave the detailled
dialog checks to TestSuperuser and pixel tests, and go back to just
ensuring that the expected title text is somewhere in the dialog.

test/common/testlib.py

@@ -940,12 +940,12 @@ def become_superuser(
             user = "root" if "suse" in self.machine.image else "admin"
 
         if passwordless:
-            self.wait_text("div[role=dialog] .pf-v5-c-modal-box__title", "Administrative access")
+            self.wait_in_text("div[role=dialog]", "Administrative access")
             self.wait_in_text("div[role=dialog] .pf-v5-c-modal-box__body", "You now have administrative access.")
             # there should be only one ("Close") button
             self.click("div[role=dialog] .pf-v5-c-modal-box__footer button")
         else:
-            self.wait_text("div[role=dialog] .pf-v5-c-modal-box__title", "Switch to administrative access")
+            self.wait_in_text("div[role=dialog]", "Switch to administrative access")
             self.wait_in_text("div[role=dialog]", f"Password for {user}:")
             self.set_input_text("div[role=dialog] input", password or "foobar")
             self.click("div[role=dialog] button.pf-m-primary")
@@ -960,7 +960,7 @@ def drop_superuser(self) -> None:
         self.switch_to_top()
 
         self.open_superuser_dialog()
-        self.wait_text("div[role=dialog] .pf-v5-c-modal-box__title", "Switch to limited access")
+        self.wait_in_text("div[role=dialog]", "Switch to limited access")
         self.click("div[role=dialog] button.pf-m-primary")
         self.wait_not_present("div[role=dialog]")
         self.check_superuser_indicator("Limited access")