public function test_doing_filter() {
    global $wp_current_filter;
    $wp_current_filter = array(); // Set to an empty array first.

    $this->assertFalse( doing_filter() );            // No filter is passed in, and no filter is being processed.
    $this->assertFalse( doing_filter( 'testing' ) ); // Filter is passed in but not being processed.

    $wp_current_filter[] = 'testing';

    $this->assertTrue( doing_filter() );                    // No action is passed in, and a filter is being processed.
    $this->assertTrue( doing_filter( 'testing' ) );         // Filter is passed in and is being processed.
    $this->assertFalse( doing_filter( 'something_else' ) ); // Filter is passed in but not being processed.

    $wp_current_filter = array();
  }