Fix docstrings test not working after stdlib change

Author: cknitt

scripts/test.js

@@ -159,6 +159,10 @@ if (runtimeDocstrings) {
       "generated_mocha_test.res",
     );
 
+    // Remove existing `generated_mocha_test.res`, as it might not compile anymore
+    // due to changes in stdlib.
+    await fs.promises.rm(generated_mocha_test_res);
+
     await execClean([], {
       cwd: docstringTestDir,
       stdio: "inherit",