Replace mailgun adapter with mock adapter (#7321)

spec/support/MockMailAdapter/index.js

@@ -0,0 +1,15 @@
+/**
+ * A mock mail adapter for testing.
+ */
+class MockMailAdapter {
+  constructor(options = {}) {
+    if (options.throw) {
+      throw 'MockMailAdapterConstructor';
+    }
+  }
+  sendMail() {
+    return 'MockMailAdapterSendMail';
+  }
+}
+
+module.exports = MockMailAdapter;