Test environment stability improvements (#703)

* Add climate_control gem and test helper

* Replace ENV mods in upgrades test

* Replace ENV mods in registrations test

* Remove ENV references in hostings controller

* Update ENV refs in mailer test

* ActiveStorage cleanup

* Consolidate queue config so appropriate adapter runs in test environment

* Make test environment more explicit

* Centralize self hosting config

* Remove flaky system test

test/controllers/registrations_controller_test.rb

@@ -25,7 +25,7 @@ class RegistrationsControllerTest < ActionDispatch::IntegrationTest
   end
 
   test "create when hosted requires an invite code" do
-    in_invited_app do
+    with_env_overrides REQUIRE_INVITE_CODE: "true" do
       assert_no_difference "User.count" do
         post registration_url, params: { user: {
           email: "john@example.com",
@@ -51,13 +51,4 @@ class RegistrationsControllerTest < ActionDispatch::IntegrationTest
       end
     end
   end
-
-  private
-
-  def in_invited_app
-    ENV["REQUIRE_INVITE_CODE"] = "true"
-    yield
-  ensure
-    ENV["REQUIRE_INVITE_CODE"] = nil
-  end
 end