DEV: Change to show failed tests only Ember tests (#14335)

This commit sets `tap_failed_tests_only` to `true` in our testem config, so now only the failing tests will show in our GitHub CI Ember test runs, which saves developers from having to hunt through all of the passing tests using GitHub's janky console output scrollback.

app/assets/javascripts/discourse/testem.js

@@ -3,6 +3,7 @@ module.exports = {
   disable_watching: true,
   launch_in_ci: ["Chrome", "Firefox", "Headless Firefox"], // Firefox is old ESR version, Headless Firefox is up-to-date evergreen version
   launch_in_dev: ["Chrome"],
+  tap_failed_tests_only: true,
   parallel: 1, // disable parallel tests for stability
   browser_args: {
     Chrome: [