Build/Test Tools: Add ms-files test group to sub-task list.

Previously, the `ms-files` test group would not run in any automatic configuration. This ensures it is always run as part of the test suite in Travis CI.

Props netweb.
Fixes #41698.


git-svn-id: https://develop.svn.wordpress.org/trunk@41659 602fd350-edb4-49c9-b593-d223f7449a82

Gruntfile.js

@@ -464,6 +464,10 @@ module.exports = function(grunt) {
 				cmd: 'phpunit',
 				args: ['--verbose', '-c', 'tests/phpunit/multisite.xml']
 			},
+			'ms-files': {
+				cmd: 'phpunit',
+				args: ['--verbose', '-c', 'tests/phpunit/multisite.xml', '--group', 'ms-files']
+			},
 			'external-http': {
 				cmd: 'phpunit',
 				args: ['--verbose', '-c', 'phpunit.xml.dist', '--group', 'external-http']