update tests for new smart object scaling

In PS <= 15.0, PSD smart objects were always scaled in pixel space, which led to fuzzy vectors.

In PS >= 15.1, PSD smart objects will now be scaled in a vector-preserving way.

This PR does two things:

1. Tests that scale smart objects are updated for the new scaling
2. A new test, `smart-object-pixel-scaling` is added to test the new configuration option being added in adobe-photoshop/generator-assets#242

Note that one test will still fail when these automated tests are run:

```
failed: scaled-layer-effects-2 - 1 error(s)
   file square50.png has a comparison metric of 13104.8 > 0
```

This failing test is due to adobe-photoshop/generator-core#211
That bug really needs to be fixed, but can be ignored for the purposes of merging this PR.