[js-fuzzer] Fix broken db tester

This was missing in one of the last refactorings.

No-Try: true
Bug: chromium:1044942
Change-Id: I2c6bfc75251fad61f35a75afec3a1b2682175d68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3127705Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76589}

tools/clusterfuzz/js_fuzzer/test_db.js

@@ -29,7 +29,6 @@ function main() {
     return;
   }
 
-  const loader = new sourceHelpers.V8SourceLoader();
   const mutateDb = new db.MutateDb(program.input_dir);
   const mutator = new crossOverMutator.CrossOverMutator(
       { MUTATE_CROSSOVER_INSERT: 1.0, testing: true }, mutateDb);
@@ -47,7 +46,7 @@ function main() {
         () => { return expression; });
     // Use a source that will try to insert one statement, allowing
     // super.
-    const source = loader.load(
+    const source = sourceHelpers.loadSource(
         __dirname,
         'test_data/regress/build_db/cross_over_mutator_class_input.js');
     try {