public function apply_testing_filter() {
    $this->apply_testing_filter = true;

    $this->assertTrue( doing_filter() );
    $this->assertTrue( doing_filter( 'testing' ) );
    $this->assertFalse( doing_filter( 'something_else' ) );
    $this->assertFalse( doing_filter( 'testing_nested' ) );

    add_filter( 'testing_nested', array( $this, 'apply_testing_nested_filter' ) );
    $this->assertTrue( has_action( 'testing_nested' ) );
    $this->assertSame( 10, has_action( 'testing_nested', array( $this, 'apply_testing_nested_filter' ) ) );

    apply_filters( 'testing_nested', '' );

    // Make sure it ran.
    $this->assertTrue( $this->apply_testing_nested_filter );

    $this->assertFalse( doing_filter( 'testing_nested' ) );
    $this->assertFalse( doing_filter( 'testing_nested' ) );
  }