Auto merge of #9589 - ehuss:testsuite-diff, r=alexcrichton

Enhancements to testsuite error output.

This includes several changes to the testsuite error reporting in theory to help make it easier to see differences when the test output changes. The key change is to add a Myers diff with a little colored output to highlight the differences. Here is an example:

<img width="666" alt="image" src="https://user-images.githubusercontent.com/43198/122311381-e8d43580-cec6-11eb-81b4-e2675f10d6ba.png">

The rest of the changes here are various refactorings to try to clean up the diffing code.  It ended up being far more changes than I was intending, but I did try to split things into commits to separate them.

Author: bors

crates/cargo-test-support/Cargo.toml

@@ -16,9 +16,11 @@ filetime = "0.2"
 flate2 = { version = "1.0", default-features = false, features = ["zlib"] }
 git2 = "0.13.16"
 glob = "0.3"
+itertools = "0.10.0"
 lazy_static = "1.0"
 remove_dir_all = "0.5"
 serde_json = "1.0"
 tar = { version = "0.4.18", default-features = false }
+termcolor = "1.1.2"
 toml = "0.5.7"
 url = "2.2.2"