test: add tests for Server model

Gemfile.lock

@@ -289,6 +289,8 @@ GEM
       sentry-ruby (~> 5.8.0)
     sentry-ruby (5.8.0)
       concurrent-ruby (~> 1.0, >= 1.0.2)
+    shoulda-matchers (6.1.0)
+      activesupport (>= 5.2.0)
     sprockets (4.2.0)
       concurrent-ruby (~> 1.0)
       rack (>= 2.2.4, < 4)
@@ -366,6 +368,7 @@ DEPENDENCIES
   secure_headers
   sentry-rails
   sentry-ruby
+  shoulda-matchers
   timecop
   turbolinks (~> 5)
   uglifier (>= 1.3.0)