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;

spec/support/MockMailAdapter/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "mock-mail-adapter",
+  "version": "1.0.0",
+  "description": "Mock mail adapter for tests.",
+  "main": "index.js"
+}