Bug 1956080 - Use async experiment cleanup in test_commands_closetab.js r=markh

ExperimentManager.unenroll() is now effectively async when running in a
browser test (because we are now executing SQL queries during
unenrollment) so all cleanup functions that trigger unenrollment are
async as well.

Differential Revision: https://phabricator.services.mozilla.com/D250558

services/fxaccounts/tests/xpcshell/test_commands_closetab.js

@@ -92,8 +92,8 @@ add_task(async function test_closetab_isDeviceCompatible() {
   // Feature successfully disabled
   Assert.ok(!closeTab.isDeviceCompatible(device));
 
-  doExperimentCleanup();
-  cleanup();
+  await doExperimentCleanup();
+  await cleanup();
 });
 
 add_task(async function test_closetab_send() {