test: add tests for message unqueueing

This adds a comprehensive set of tests for the message unqueueing service.

Additionally, it improves how message databases are used for testing environments.

spec/app/models/outgoing_message_prototype_spec.rb

@@ -3,21 +3,20 @@
 require "rails_helper"
 
 describe OutgoingMessagePrototype do
+  let(:server) { create(:server) }
   it "should create a new message" do
-    with_global_server do |server|
-      domain = create(:domain, owner: server)
-      prototype = OutgoingMessagePrototype.new(server, "127.0.0.1", "TestSuite", {
-        from: "test@#{domain.name}",
-        to: "test@example.com",
-        subject: "Test Message",
-        plain_body: "A plain body!"
-      })
+    domain = create(:domain, owner: server)
+    prototype = OutgoingMessagePrototype.new(server, "127.0.0.1", "TestSuite", {
+      from: "test@#{domain.name}",
+      to: "test@example.com",
+      subject: "Test Message",
+      plain_body: "A plain body!"
+    })
 
-      expect(prototype.valid?).to be true
-      message = prototype.create_message("test@example.com")
-      expect(message).to be_a Hash
-      expect(message[:id]).to be_a Integer
-      expect(message[:token]).to be_a String
-    end
+    expect(prototype.valid?).to be true
+    message = prototype.create_message("test@example.com")
+    expect(message).to be_a Hash
+    expect(message[:id]).to be_a Integer
+    expect(message[:token]).to be_a String
   end
 end