Transform sub/sub directories into a proper dbprefix

Enables a test case similar to the following:

```
Given a WP install in 'subdir1'
And I run `mkdir -p sub/sub`
And a WP install in 'sub/subdir2'
And a WP install in 'sub/sub/subdir3'
```

Author: danielbachhuber

features/bootstrap/FeatureContext.php

@@ -318,7 +318,7 @@ public function download_wp( $subdir = '' ) {
 
 	public function create_config( $subdir = '' ) {
 		$params = self::$db_settings;
-		$params['dbprefix'] = $subdir ?: 'wp_';
+		$params['dbprefix'] = $subdir ? preg_replace( '#[^a-zA-Z\_0-9]#', '_', $subdir ) : 'wp_';
 
 		$params['skip-salts'] = true;
 		$this->proc( 'wp core config', $params, $subdir )->run_check();