improved test error reporting

Wanted to make it clearer what the failures were from the new @michalrus tests, so I changed `assert` to `expectResult` everywhere. Failures now explain what happened:

```
[info] - can use attributes in xml literals *** FAILED ***
[info]   result.errors
[info] Expected List(), but got List(var is disabled) (XmlLiteralTest.scala:20)
```

Any objections?