Optionally run ignored tests

[djc]

In Quinn, we have one test which we ignore, so that we don't run it locally; but we do want to run it in CI. Would be nice if this was supported in the stages templates (as a parameter?).

This basically involves passing --ignored to cargo test.

https://doc.rust-lang.org/book/ch11-02-running-tests.html#ignoring-some-tests-unless-specifically-requested

[jonhoo]

#42 adds a test_ignored parameter to stages, tests, and test. I also added single_threaded should you want to run your tests with --test-threads=1.