fix that js/file-system-race could have FPs related to loops

javascript/ql/test/query-tests/Security/CWE-367/tst.js

@@ -41,3 +41,8 @@ const filePath3 = createFile();
 if (fs.existsSync(filePath3)) {
   fs.readFileSync(filePath3); // OK - a read after an existence check is OK 
 }
+
+const filePath4 = createFile();
+while(Math.random() > 0.5) {
+    fs.open(filePath4); // OK - it is only ever opened here.
+}