cache-flush check: Return early if a match is found

We currently scan all the PHP files in the target WP content dir looking
for `wp_cache_flush()` uses, however we are not currently using the
results in a meaninful way so we can just probably return early if the
current check found a match.

If there are plans to list the matched files, we can probably attempt to
use some more performan alternatives (grep if available for example)
which should be likely faster than a PHP based implementation.

See #178

inc/checks/class-cache-flush.php

@@ -23,6 +23,10 @@ public function run() {
 
 		foreach ( $iterator as $file ) {
 			$this->check_file( $file );
+			if ( ! empty( $this->_matches ) ) {
+				// we are currently interested whether there's a use of wp_cache_flush() or not.
+				break;
+			}
 		}
 
 		if ( empty( $this->_matches ) ) {