Racket runner doesn't include common packages

[ruliana]

For the exercism "meetup", it's not uncommon to require the gregor 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). However, the runner doesn't have this dependency and fails.

Related to that, other common packages could be part of the runner, like threading and data/collections.

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

[BNAndras]

76 people attempted meetup with only half completing it so there might be some use for gregor if it it simplifies the exercise . However, I'm reluctant to break the existing solutions if we can't handle in the test files whether students use gregor or the standard library. Feel free to make a PR in https://github.com/exercism/racket showing how we could handle both situations.

Otherwise, I think threading and data/collections is likely fine to add to the runner if they don't increase the file size greatly.