Bug 1781451 - Add a missing await in the test r=asuth

PushService.init is an async function waiting on a queue, thus it should be awaited to make sure the mock service registers before `PushService.unregister`.

Differential Revision: https://phabricator.services.mozilla.com/D247795

dom/push/test/xpcshell/test_unregister_invalid_json.js

@@ -46,7 +46,7 @@ add_task(async function test_unregister_invalid_json() {
   let unregisterPromise = new Promise(
     resolve => (unregisterDone = after(2, resolve))
   );
-  PushService.init({
+  await PushService.init({
     serverURI: "wss://push.example.org/",
     db,
     makeWebSocket(uri) {

dom/push/test/xpcshell/xpcshell.toml

@@ -96,10 +96,6 @@ run-sequentially = "This will delete all existing push subscriptions."
 ["test_unregister_error.js"]
 
 ["test_unregister_invalid_json.js"]
-run-sequentially = "very high failure rate in parallel"
-skip-if = [
-  "os == 'linux' && os_version == '18.04' && processor == 'x86_64' && tsan", # Bug 1933914
-]
 
 ["test_unregister_not_found.js"]