MDL-63977 Behat: Allow Behat testing of the Moodle mobile app

This change allows you to write and run Behat tests that cover the
mobile app. These should have the @app tag. They will be run in the
Chrome browser using an Ionic server on the local machine.

See config-dist.php for configuration settings, or full docs here:
https://docs.moodle.org/dev/Acceptance_testing_for_the_mobile_app

lib/behat/classes/behat_command.php

@@ -219,6 +219,12 @@ class behat_command {
             return BEHAT_EXITCODE_CONFIG;
         }
 
+        // If app config is supplied, check the value is correct.
+        if (!empty($CFG->behat_approot) && !file_exists($CFG->behat_approot . '/ionic.config.json')) {
+            self::output_msg(get_string('errorapproot', 'tool_behat'));
+            return BEHAT_EXITCODE_CONFIG;
+        }
+
         return 0;
     }