sorbet: Bump `test/support/helper` files to `typed: true`

- Add `T.bind`s for RSpec example groups and matcher test helpers.
- For the undefined predicate method raising NoMethodError, trick Sorbet
  into not flagging it as an error by using `public_send` instead of
  direct method call - because, correctly, the method does not exist but
  the test is already verifying that.