MDL-40475 add alternative component cache location and other improvements

Improvements include:
* Alternative location might be useful when server administrator wants to maintain
  a local copy of component cache instead of using shared $CFG->cachedir.
* Component caching is now enabled in behat tests which should improve performance.
* Standardised ignoring of component caching.
* Fixed debug mode in ABORT_AFTER_CONFIG scripts.
* General documentation improvements.

lib/phpunit/bootstrap.php

@@ -40,6 +40,10 @@ if (ini_get('opcache.enable') and strtolower(ini_get('opcache.enable')) !== 'off
     }
 }
 
+if (!defined('IGNORE_COMPONENT_CACHE')) {
+    define('IGNORE_COMPONENT_CACHE', true);
+}
+
 require_once(__DIR__.'/bootstraplib.php');
 require_once(__DIR__.'/../testing/lib.php');
 require_once(__DIR__.'/classes/autoloader.php');