aquaregia_test: Better error message matching

[abhinav]

A major piece of debt in aquaregia_test (which verifies error messages for failing test cases)
is that it just searches for N instance of error X in the output.
We can do better: we should assert error messages at specific positions.

One way we could do this is how analysistest expects error messages: with // want comments next to where the errors are expected.

We should also move all the test cases to a testdata/ folder so we don't have to tag them with the failing constraint.