my $mock_fs = Test::MockFileSys->new;
$mock_fs->file('/foo/bar'); # push $self->{foo}->@*, Test::MockFile->file('/foo/bar');
$mock_fs->file('/foo/baz');
$mock_fs->dir('/foo');
$mock_fs->file('/foo/bar', '...'); # Retrieves existing object or creates new if missing.