testsuite: Switch to colored diffs with Myers diff.

Author: ehuss

crates/cargo-test-support/Cargo.toml

@@ -21,5 +21,6 @@ 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"