test: Wait for systemd service memory to appear

Memory usage is polled, it does not appear immediately after starting
the unit. This caused TestServices.testBasicSession to flake.

test/verify/check-system-services

@@ -618,6 +618,7 @@ ExecStart=/bin/awk -f /tmp/mem-hog{params}.awk
             self.run_systemctl(user, "start mem-hog.service")
             # If the test fails before we stop the mem-hog the next test run will not get the correct memory usage here
             self.addCleanup(self.run_systemctl, user, "stop mem-hog.service || true")
+            b.wait_visible("#memory .pf-c-description-list__text")
             initial_memory = float(b.text("#memory .pf-c-description-list__text").split(" ")[0])
             self.assertGreater(initial_memory, 0.5)
             m.execute(f"touch /tmp/continue{params}")