Run tests one by one @ GHA

This workflow just finds all the individual
tests available in core (all the test_ methods
in _test.php file) and runs them one by one,
with a total isolation, because each one is
executed by a different PHPUnit run.

Any test ending with error, will be reported
as part of the output.

Note that we are not using PHPUnit's own
isolation here, just running them one by one.

.github/workflows/onebyone.yml

@@ -19,6 +19,13 @@ name: One by One Testing
 # own isolation or any other option.
 
 on:
+  push:
+    branches-ignore:
+      - main
+      - master
+      - MOODLE_[0-9]+_STABLE
+    tags-ignore:
+      - v[0-9]+.[0-9]+.[0-9]+*
   workflow_dispatch:
     inputs:
       phpunit_extra_options: