Racket runner doesn't include common packages

For the exercism "meetup", it's not uncommon to require the [`gregor`](https://docs.racket-lang.org/gregor/index.html) library as the Racket standard library for dates is quite bare of functionalities (The package is even mentioned in a side box in the [standard documentation](https://docs.racket-lang.org/reference/time.html#%28mod-path._racket%2Fdate%29)). However, the runner doesn't have this dependency and fails.

Related to that, other common packages could be part of the runner, like [`threading`](https://docs.racket-lang.org/threading/index.html) and [`data/collections`](https://docs.racket-lang.org/collections/index.html).

(Admittedly, threading and data/collections are more a convenience)