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/tests/mod.rs

@@ -0,0 +1,28 @@
+use crate::eval::interpret;
+
+use test_generator::test_resources;
+
+// eval-okay-* tests contain a snippet of Nix code, and an expectation
+// of the produced string output of the evaluator.
+//
+// These evaluations are always supposed to succeed, i.e. all snippets
+// are guaranteed to be valid Nix code.
+#[test_resources("src/tests/nix_tests/eval-okay-*.nix")]
+fn eval_okay(code_path: &str) {
+    let base = code_path
+        .strip_suffix("nix")
+        .expect("test files always end in .nix");
+    let exp_path = format!("{}exp", base);
+
+    let code = std::fs::read_to_string(code_path).expect("should be able to read test code");
+    let exp = std::fs::read_to_string(exp_path).expect("should be able to read test expectation");
+
+    let result = interpret(&code).expect("evaluation of eval-okay test should succeed");
+    let result_str = format!("{}", result);
+
+    assert_eq!(
+        exp.trim(),
+        result_str,
+        "result value (and its representation) must match expectation"
+    );
+}