New linter to enforce specific `regexp` error message in `expect_error`

[nicklamiller]

A linter that enforces calls to testthat::expect_error have a non-NULL regexp argument would be helpful to ensure tests that expect failure only fail with the specified error message and not for some unanticipated reason. This is analogous to pytest-raises-too-broad (PT011) in ruff.

This linter (or maybe separate linters) could naturally enforce a specific regexp for other testthat functions that have a regexp argument: expect_error, expect_warning, expect_message, and expect_condition.

[MichaelChirico]

I go back & forth on this one a little bit. One reason to leave a NULL regexp is if your error message is produced by another package/system call. Relying on the exact wording of someone else's code is fragile (& classed errors are very much not the norm in R, unfortunately, whereas they're more typical in Python). There are workarounds, but sometimes more hassle than it's worth.

Overall, I think it's fine to have this linter, but would want to mention this caution in the documentation; nolint may be appropriate.