Build/Test Tools: Expand Slack notifications for GitHub Actions.

This expands Slack notifications to include success, cancelled, and “fixed” GitHub Action workflow run outcomes in addition to failures.

A “fixed” outcome occurs when the previous run for a workflow failed and the current one succeeds. This matches the behavior that was native to TravisCI by setting `on_success` for notifications to `change`.

The message details and where each outcome is posted is controlled by Slack workflows.

The Slack notification logic has also been pulled into a separate workflow to prevent repeating code in every workflow.

See #52644.

git-svn-id: https://develop.svn.wordpress.org/trunk@51535 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/functions.php

@@ -27,7 +27,7 @@ class Tests_Functions extends WP_UnitTestCase {
 		$this->assertSame( array(), wp_parse_args( $a ) );
 		$b = array(
 			'_baba' => 5,
-			'yZ'    => 'baba',
+			'yZ'    => 'bab',
 			'a'     => array( 5, 111, 'x' ),
 		);
 		$this->assertSame(