feat(tvix/glue): introduce test suite for context strings

This is an additional test suite on the top of the Nix ones
for context strings matters.

It already smoked out multiple mistakes and potential bugs and non-deterministic result from the evaluator.

It uses a similar technology as the one in the tvix-eval albeit we instantiate a fully fledged evaluator
with in-memory store.

We copy the files instead of symlinking them because crates are built in
isolation, so symlinks cannot work.

Change-Id: I63ae225ce4f83c6e2c8ccd60d779c2f8eb9d08fb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10619
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/glue/src/lib.rs

@@ -5,6 +5,9 @@ pub mod tvix_build;
 pub mod tvix_io;
 pub mod tvix_store_io;
 
+#[cfg(test)]
+mod tests;
+
 /// Tell the Evaluator to resolve `<nix>` to the path `/__corepkgs__`,
 /// which has special handling in [tvix_io::TvixIO].
 /// This is used in nixpkgs to import `fetchurl.nix` from `<nix>`.