[test] Fix wasm2js stale check failure

I tried to exclude wasm2js asserts tests from `check_for_stale_files` in
 WebAssembly#6164, but ended up doing it incorrectly. The file I checked for was
`wasm2js.wast.asserts`, while the output I should have excluded was
`wasm2js.asserts.js`. This fixes the code so we now check the prefix and
not the filename.

This is currently breaking the main branch, which was caused by me
landing multiple PRs at once. Sorry for the breakage. Landing this will
fix it.

scripts/test/wasm2js.py

@@ -41,9 +41,9 @@ def check_for_stale_files():
 
     all_files = os.listdir(shared.get_test_dir('wasm2js'))
     for f in all_files:
-        if f in assert_tests:
-            continue
         prefix = f.split('.')[0]
+        if prefix in [t.split('.')[0] for t in assert_tests]:
+          continue
         if prefix not in all_tests:
             shared.fail_with_error('orphan test output: %s' % f)