Update `libtest-mimic` to 0.5.0

[LukasKalbertodt]

I just released a new version of libtest-mimic. This is one of the bigger projects using it, so as a sanity check whether the new version still works for real world users, I just did this update. It allowed to remove some of the code here as the API changed a bit and panics are already caught by libtest-mimic. Also, quite a bit indirect dependencies are not needed anymore.

Two things to note:

• libtest-mimic requires Rust 1.58. I don't know if that's fine for your MSRV policy.
• I do not know about dhall at all and did not test anything except compiling the tests. Please review carefully and/or test.

[Nadrieril]

Hey! Thank you for taking the time, this is indeed a nicer API! Honored that I'm one of the bigger projects using this, it's a neat library. I tested the changes and everything looks good.

For some reason I can't push to this PR (maybe my setup is broken or your master branch blocks direct pushes), so I made https://github.com/Nadrieril/dhall-rust/pull/234 as a replacement