Add additional mocks needed for tests in new versions of @actions (#343)

Tests started failing after @actions was bumped in
https://github.com/hashicorp/vault-action/pull/334

src/auth.test.js

@@ -1,7 +1,12 @@
 jest.mock('got');
 jest.mock('@actions/core');
 jest.mock('@actions/core/lib/command');
-jest.mock("fs")
+jest.mock('fs', () => ({
+    stat: jest.fn().mockResolvedValue(null),
+    promises: {
+        access: jest.fn().mockResolvedValue(null),
+    }
+}));
 
 const core = require('@actions/core');
 const got = require('got');