public function test_simple_filter() {
    $a   = new MockAction();
    $tag = __FUNCTION__;
    $val = __FUNCTION__ . '_val';

    add_filter( $tag, array( $a, 'filter' ) );
    $this->assertSame( $val, apply_filters( $tag, $val ) );

    // Only one event occurred for the hook, with empty args.
    $this->assertSame( 1, $a->get_call_count() );
    // Only our hook was called.
    $this->assertSame( array( $tag ), $a->get_tags() );

    $argsvar = $a->get_args();
    $args    = array_pop( $argsvar );
    $this->assertSame( array( $val ), $args );
  }