Bug 1956080 - Use async experiment cleanup in browser_contentrelevancy_nimbus.js r=nanj

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/D250559

toolkit/components/contentrelevancy/tests/browser/browser_contentrelevancy_nimbus.js

@@ -63,7 +63,7 @@ add_task(async function test_NimbusIntegration_enable() {
     "The timer callback should be called"
   );
 
-  doExperimentCleanup();
+  await doExperimentCleanup();
   gSandbox.restore();
 });
 
@@ -94,6 +94,6 @@ add_task(async function test_NimbusIntegration_disable() {
     "Should not register timer"
   );
 
-  doExperimentCleanup();
+  await doExperimentCleanup();
   gSandbox.restore();
 });