test: fix comparing element style attribute value
test: fix comparing element style attribute value

The order of propeties in the style attribute value can change so it
is formatted and sorted before the comparison.

The change unveils the fact the updated radium has the follwing prefixer
issue fixed.

From the test output:

```
- actual, + expected

  -display:-moz-box
  -display:-ms-flexbox
  -display:-webkit-box
  -display:-webkit-flex
  -display:flex
  +display:-webkit-box,-moz-box,-ms-flexbox,-webkit-flex,flex
```

apiaryio/attributes-kit,apiaryio/attributes-kit,apiaryio/attributes-kit