tool_forcedcache: Add tearDown method to runtests

Resolves error "Property 'tmpdir' defined in 'tool_forcedcache_cache_config_test' was not reset after the test!"

Author: tatiana-gl

tests/cache_config_test.php

@@ -46,6 +46,11 @@ public function copy_to_valid_config_location(string $source): string {
         return realpath($dest);
     }
 
+    protected function tearDown(): void {
+        $this->tmpdir = null;
+        parent::tearDown();
+    }
+
     public function test_read_config_file_from_invalid_path() {
         global $CFG;
         $this->resetAfterTest(true);