test: add initial tests for Postal::SMTPServer::Client

spec/factories/http_endpoint_factory.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :http_endpoint do
+    server
+    name { "HTTP endpoint" }
+    url { "https://example.com/endpoint" }
+    encoding { "BodyAsJSON" }
+    format { "Hash" }
+  end
+end