Make it easy to run only tests for one specific core module

Problem/Motivation

The Drupal core test suite takes ~60 minutes to test Drupal 10.

This:

1. slows down and hence discourages contributions
2. costs a lot to the Drupal Association

We did this a lot while working to get the CKEditor 5 module stable. It saved countless hours, allowed for faster iterations and saved many DA dollars. It required applying this patch:

diff --git a/core/drupalci.yml b/core/drupalci.yml
index 94aa2ad22f..6b4e9a767b 100644
--- a/core/drupalci.yml
+++ b/core/drupalci.yml
@@ -15,30 +15,30 @@ build:
       # deprecated code.
       run_tests.phpunit:
         types: 'PHPUnit-Unit'
-        testgroups: '--all'
+        testgroups: '--module ckeditor5'
         suppress-deprecations: false
         halt-on-fail: false
       run_tests.kernel:
         types: 'PHPUnit-Kernel'
-        testgroups: '--all'
+        testgroups: '--module ckeditor5'
         suppress-deprecations: false
         halt-on-fail: false
       run_tests.build:
         # Limit concurrency due to disk space concerns.
         concurrency: 15
         types: 'PHPUnit-Build'
-        testgroups: '--all'
+        testgroups: '--module ckeditor5'
         suppress-deprecations: false
         halt-on-fail: false
       run_tests.functional:
         types: 'PHPUnit-Functional'
-        testgroups: '--all'
+        testgroups: '--module ckeditor5'
         suppress-deprecations: false
         halt-on-fail: false
       run_tests.javascript:
         concurrency: 15
         types: 'PHPUnit-FunctionalJavascript'
-        testgroups: '--all'
+        testgroups: '--module ckeditor5'
         suppress-deprecations: false
         halt-on-fail: false
       # Run nightwatch testing.
diff --git a/core/package.json b/core/package.json
index c14f58a08c..cc55f438eb 100644
--- a/core/package.json
+++ b/core/package.json
@@ -18,7 +18,7 @@
     "lint:css": "stylelint \"**/*.css\"",
     "lint:css-checkstyle": "stylelint \"**/*.css\" --custom-formatter ./node_modules/stylelint-checkstyle-formatter/index.js",
     "lint:yaml": "node ./node_modules/eslint/bin/eslint.js --ext .yml .",
-    "test:nightwatch": "node -r dotenv-safe/config ./node_modules/.bin/nightwatch --config ./tests/Drupal/Nightwatch/nightwatch.conf.js",
+    "test:nightwatch": "node -r dotenv-safe/config ./node_modules/.bin/nightwatch --config ./tests/Drupal/Nightwatch/nightwatch.conf.js --tag ckeditor5",
     "prettier": "prettier --write \"./**/*.js\"",
     "spellcheck": "cspell",
     "spellcheck:make-drupal-dict": "rm -f misc/cspell/dictionary.txt && touch misc/cspell/dictionary.txt && yarn -s spellcheck:core --unique --wordsOnly | tr '[:upper:]' '[:lower:]' | tr -d \\\\\\\\ | LC_ALL=C sort -u -o misc/cspell/dictionary.txt",

But it's not reasonable to expect all Drupal core contributors to figure this out.

Steps to reproduce

Post a patch or an MR.

Proposed resolution

Many (most?) changes to Drupal core actually apply only to a single module.

So … why not make it as simple as possible to run only the tests for a particular Drupal core module? Yes, soonish we'll transition from Drupal CI to GitLab CI. But even before then, there's a very low-cost, low-effort way to make it easie. What if you'd only have to change this in core/drupalci.yml:

_phpunit_testgroups_to_execute: &testgroups
  # Default: all of Drupal core's test suite runs.
  testgroups: '--all'
  # Alternative: run only the tests for one particular module.
  # testgroups: '--module ckeditor5'

to this:

_phpunit_testgroups_to_execute: &testgroups
  # Default: all of Drupal core's test suite runs.
  #testgroups: '--all'
  # Alternative: run only the tests for one particular module.
  testgroups: '--module ckeditor5'

(comment one line, uncomment another)

Note: this will not eliminate all nightwatch tests from running (because that does not use PHPUnit and requires tweaking core/package.json), which takes ~6.5 minutes, but tweaking that can be a follow-up improvement. This alone cuts ~66% off the test run time.

This alone would cut makes a huge difference.

Remaining tasks

Review

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.