lang-python: fix unparse roundtrip check in bench.ts (#583)

Fixed the unparse roundtrip check in the Python benchmark. The CST is
built from tokenized input (where comments are replaced with spaces), so
reconstructing the full source now correctly uses the tokenized input
for the prefix before the root node instead of the raw input.

Author: pdubroy

packages/lang-python/bench.ts

@@ -61,7 +61,7 @@ bench.add(
     const cst = r.getCstRoot();
     const {input: tokenizedInput} = tokenize(input);
     const root = cst as NonterminalNode;
-    const fullSource = input.slice(0, root.startIdx) + root.sourceString;
+    const fullSource = tokenizedInput.slice(0, root.startIdx) + root.sourceString;
     if (fullSource !== tokenizedInput) {
       console.error('UNPARSE MISMATCH!');
       process.exit(1);