config test: explicitly declare class property

Resolves error 'Creation of dynamic property tool_forcedcache_cache_config_test:: is deprecated'

Author: Sasha Anastasi

tests/cache_config_test.php

@@ -25,6 +25,13 @@
  */
 class tool_forcedcache_cache_config_test extends \core_phpunit\testcase {
 
+    /**
+     * Temporary directory for loading config into
+     *
+     * @var string
+     */
+    private $tmpdir;
+
     /**
      * We need to load the config files outside of the $CFG->dirroot, so it
      * will be copied out as part of these tests.