tests: `setupExercismRepo`: could retry if `git checkout` fails

Erik saw when running `nimble test` :

> [Suite] generate
> fatal: reference is not a tree: 91ccf91940f32aff3726c772695b2de167d8192a
>
> Error when running `git -C /path/to/exercism/configlet/tests/.test_elixir_track_repo checkout --force 91ccf91940f32aff3726c772695b2de167d8192a`

Which was resolved by deleting the `tests/.test_elixir_track_repo` directory and rerunning `nimble test`.

This was probably due to a network error or keyboard interrupt during the previous clone of the repo. The checkout command should always succeed, so the tests could theoretically handle this case - trying to clone the repo again from scratch if the checkout failed.

The code is here:

https://github.com/exercism/configlet/blob/f8d86a35b92b5f5137fce931598d5ffc14fb9cac/src/exec.nim#L103-L118