add coverage for inverse futures private getter endpoints, migrate private getter methods to base class

src/util/node-support.ts

@@ -1,7 +1,8 @@
 import { createHmac } from 'crypto';
 
-export async function signMessage(message: string, secret: string): Promise<string> {
-  return createHmac('sha256', secret)
-    .update(message)
-    .digest('hex');
-};
+export async function signMessage(
+  message: string,
+  secret: string
+): Promise<string> {
+  return createHmac('sha256', secret).update(message).digest('hex');
+}