[infra] add native_test_helpers to be a valid PR title prefix (#2390)

Author: goderbauer

.github/pr-title-checker-config.json

@@ -11,6 +11,7 @@
             "[jni] ",
             "[jnigen] ",
             "[native_doc_dartifier] ",
+            "[native_test_helpers] ",
             "[native_toolchain_c] ",
             "[objective_c] ",
             "[swift2objc] ",