Run skipped/ignored tests with `--run-ignored`

Add a `--run-ignored` flag to run tests that are normally skipped:

- `--run-ignored only` — run only skipped tests
- `--run-ignored all` — run skipped tests alongside normal tests

This is useful for debugging why tests are being skipped, or for periodically checking if skip conditions are still valid.

See: https://nexte.st/docs/running/#options-and-arguments