Fix showing syntax errors in sync test files

tests/js/index.js

@@ -35,11 +35,16 @@ if (!(typeof process === 'object' && process.platform === 'win32')) {
 }
 
 // If sync is enabled, run the user tests
+let hasSync = false;
 try {
     Realm.Sync; // This will throw if Sync is disabled.
+    hasSync = true;
+} catch (e) { }
+
+if (hasSync) {
     TESTS.UserTests = require('./user-tests');
     TESTS.SessionTests = require('./session-tests');
-} catch (e) {}
+}
 
 function node_require(module) { return require(module); }