test(tazjin/rlox): Add a handful of interpreter tests

Change-Id: I32dd896d42cc73d68d73093e9cbb74b48d95e041
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2386
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/interpreter.rs

@@ -8,6 +8,9 @@ use std::sync::RwLock;
 // Implementation of built-in functions.
 mod builtins;
 
+#[cfg(test)]
+mod tests;
+
 // Tree-walk interpreter
 
 // Representation of all callables, including builtins & user-defined