refactor(tvix/eval): use pretty_assertions for tests

This makes for much more readable output especially when long strings
are involved.

Change-Id: I43dd73a0480535d7181a760788c42883a9b083f8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6229
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/Cargo.toml

@@ -16,6 +16,7 @@ tabwriter = { version = "1.2", optional = true }
 [dev-dependencies]
 criterion = "0.3.6"
 test-generator = "0.3.0"
+pretty_assertions = "1.2.1"
 
 [features]
 # Enables running the Nix language test suite from the original C++