feat(tvix/tests): check in Nix' language test suite

This adds scaffolding code for running the Nix language test suite.

The majority of eval-okay-* tests should eventually be runnable as-is
by Tvix, however the eval-fail-* tests might not as we intend to have
more useful error messages than upstream Nix.

Change-Id: I4f3227f0889c55e4274b804a3072850fb78dd1bd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6126
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/main.rs

@@ -12,6 +12,9 @@ mod opcode;
 mod value;
 mod vm;
 
+#[cfg(test)]
+mod tests;
+
 fn main() {
     let mut args = env::args();
     if args.len() > 2 {
@@ -47,8 +50,8 @@ fn run_prompt() {
 }
 
 fn run(code: String) {
-    match eval::interpret(code) {
-        Ok(result) => println!("=> {}", result),
+    match eval::interpret(&code) {
+        Ok(result) => println!("=> {} :: {}", result, result.type_of()),
         Err(err) => eprintln!("{}", err),
     }
 }