Add tests for custom attributes in scarb

[Arcticae]

Attributes like allow(...) (or whichever ones we use/add in the lint plugin) should be tested in scarb, so that we make sure it actually compiles.

Ideally upon adding new allowed ones - we have test for that too (i.e. something like fixture test, crashing upon supported attribute appears but the snapshot is not updated) so that we know they're actually being enabled on the scarb side.