[test] Fix wasm2js stale check failure (WebAssembly#6168)

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.

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)