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/lib/postal/message_db/database_spec.rb

@@ -4,7 +4,8 @@ require "rails_helper"
 
 describe Postal::MessageDB::Database do
   context "when provisioned" do
-    subject(:database) { GLOBAL_SERVER.message_db }
+    let(:server) { create(:server) }
+    subject(:database) { server.message_db }
 
     it "should be a message db" do
       expect(database).to be_a Postal::MessageDB::Database

spec/lib/postal/message_parser_spec.rb

@@ -3,25 +3,23 @@
 require "rails_helper"
 
 describe Postal::MessageParser do
+  let(:server) { create(:server) }
+
   it "should not do anything when there are no tracking domains" do
-    with_global_server do |server|
-      expect(server.track_domains.size).to eq 0
-      message = create_plain_text_message(server, "Hello world!", "test@example.com")
-      parser = Postal::MessageParser.new(message)
-      expect(parser.actioned?).to be false
-      expect(parser.tracked_links).to eq 0
-      expect(parser.tracked_images).to eq 0
-    end
+    expect(server.track_domains.size).to eq 0
+    message = create_plain_text_message(server, "Hello world!", "test@example.com")
+    parser = Postal::MessageParser.new(message)
+    expect(parser.actioned?).to be false
+    expect(parser.tracked_links).to eq 0
+    expect(parser.tracked_images).to eq 0
   end
 
   it "should replace links in messages" do
-    with_global_server do |server|
-      message = create_plain_text_message(server, "Hello world! http://github.com/atech/postal", "test@example.com")
-      create(:track_domain, server: server, domain: message.domain)
-      parser = Postal::MessageParser.new(message)
-      expect(parser.actioned?).to be true
-      expect(parser.new_body).to match(/^Hello world! https:\/\/click\.#{message.domain.name}/)
-      expect(parser.tracked_links).to eq 1
-    end
+    message = create_plain_text_message(server, "Hello world! http://github.com/atech/postal", "test@example.com")
+    create(:track_domain, server: server, domain: message.domain)
+    parser = Postal::MessageParser.new(message)
+    expect(parser.actioned?).to be true
+    expect(parser.new_body).to match(/^Hello world! https:\/\/click\.#{message.domain.name}/)
+    expect(parser.tracked_links).to eq 1
   end
 end

spec/lib/postal/smtp_server/client/finished_spec.rb

@@ -7,7 +7,7 @@ module Postal
 
     describe Client do
       let(:ip_address) { "1.2.3.4" }
-      let(:server) { GLOBAL_SERVER } # We'll use the global server instance for this
+      let(:server) { create(:server) }
       subject(:client) { described_class.new(ip_address) }
 
       let(:credential) { create(:credential, server: server, type: "SMTP") }
@@ -22,10 +22,6 @@ module Postal
         client.handle("RCPT TO: #{rcpt_to}")
       end
 
-      after do
-        server.message_db.provisioner.clean
-      end
-
       describe "when finished sending data" do
         context "when the data is larger than the maximum message size" do
           it "returns an error and resets the state" do