[fine] Many test improvements, error improvements

- Check for more error conditions
- Move to more definitive error assertions
- Simpler error messages in some cases
- Test more conditions more thoroughly, revisit old tests

fine/tests/expression/errors/class_duplicate_fields.fine

@@ -3,6 +3,5 @@ class Foo {
   x: f64;
 }
 
-// @ignore
 // @expect-errors:
-// asdfadsf
+// | 3:2: duplicate definition of field 'x'