public function test_multiple_filters() {
    $a1  = new MockAction();
    $a2  = new MockAction();
    $tag = __FUNCTION__;
    $val = __FUNCTION__ . '_val';

    // Add both filters to the hook.
    add_filter( $tag, array( $a1, 'filter' ) );
    add_filter( $tag, array( $a2, 'filter' ) );

    $this->assertSame( $val, apply_filters( $tag, $val ) );

    // Both filters called once each.
    $this->assertSame( 1, $a1->get_call_count() );
    $this->assertSame( 1, $a2->get_call_count() );
  }