integration test: test if samples marked as `skip` are failed

[eitsupi]

One thing we could do is test that the skip examples fail — that's work worked well for the other tests — it means we enable them as soon as they start working. Otherwise we often forget about them.

Originally posted by @max-sixty in #2829 (comment)

[max-sixty]

(as an example of this — here's a skip comment:

prql/prqlc/prqlc/tests/integration/queries/read_csv.prql

Line 2 in 76c4f76

# postgres:skip

)