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.

admin/tool/behat/cli/init.php

@@ -34,6 +34,7 @@ if (function_exists('opcache_reset')) {
 
 // Is not really necessary but adding it as is a CLI_SCRIPT.
 define('CLI_SCRIPT', true);
+define('CACHE_DISABLE_ALL', true);
 
 // Basic functions.
 require_once(__DIR__ . '/../../../../lib/clilib.php');