the upstream Nix test suite uses the pattern of having `.nix` and
`.exp` files for input/expected output, and with this shitty function
navigating between them is a lot simpler
Change-Id: I9d91290057521fe1e1599f69fd6b0f35e1b59960
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6058
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI