Don't run test:all as the basic test behavior

The `concurrently` invocation was set to run all `ember-try` scenarios
*every* time we run `yarn test` either locally or in CI, rather than
only when we opt into doing so with `test:all`. Switch back to doing a
single `ember test` invocation as the main `test` invocation instead.

Author: chriskrycho

package.json

@@ -36,7 +36,7 @@
     "postpublish": "yarn clean",
     "release": "release-it",
     "start": "ember serve",
-    "test": "concurrently \"npm:test:*\" --names \"test:\"",
+    "test": "ember test",
     "test:all": "ember try:each"
   },
   "peerDependencies": {