Build/Test Tools: Avoid running full PHPUnit test suite for every PHP file change.

Use `grunt watch --phpunit --group={testgroup}` to start `grunt watch` with a specific test group so that PHP file changes trigger a limited number of tests.

Props jeremyfelt, birgire for testing.
Fixes #44240.


git-svn-id: https://develop.svn.wordpress.org/trunk@43335 602fd350-edb4-49c9-b593-d223f7449a82

Gruntfile.js

@@ -1111,10 +1111,6 @@ module.exports = function(grunt) {
 					'!tests/qunit/editor/**'
 				],
 				tasks: ['qunit']
-			},
-			phpunit: {
-				files: [ '**/*.php' ],
-				tasks: [ 'phpunit:default' ]
 			}
 		}
 	});
@@ -1160,6 +1156,13 @@ module.exports = function(grunt) {
 			grunt.task.run( 'build' );
 		}
 
+		if ( 'watch:phpunit' === grunt.cli.tasks[ 0 ] || 'undefined' !== typeof grunt.option( 'phpunit' ) ) {
+			grunt.config.data._watch.phpunit = {
+				files: [ '**/*.php' ],
+				tasks: [ 'phpunit:default' ]
+			};
+		}
+
 		grunt.task.run( '_' + this.nameArgs );
 	} );