cargo-llvm-cov: document test failure modes

# If the tests are broken, it's probably for one of two reasons:

#

# 1. The version of llvm used doesn't match the expectations of rustc and/or

#    cargo-llvm-cov. This is relatively unlikely because we pull llvm out of

#    rustc's attrset, so it *should* be the right version as long as this is the

#    case.

# 2. Nixpkgs has changed its rust infrastructure in a way that causes

#    cargo-llvm-cov to misbehave under test. It's likely that even though the

#    tests are failing, cargo-llvm-cov will still function properly in actual

#    use. This has happened before, and is described [here][0] (along with a

#    feature request that would fix this instance of the problem).

#

# For previous test-troubleshooting discussion, see [here][1].

#

# [0]: https://github.com/taiki-e/cargo-llvm-cov/issues/242

# [1]: https://github.com/NixOS/nixpkgs/pull/197478

{ stdenv

, lib

, fetchurl