tests: fix test failures when running without API keys.

Fix the test suite failing when trying to run it in the default state
with no config file or API keys configured. Most source sites require
API keys or login credentials to be set in order to work. Skip these
tests when credentials aren't configured.

test/unit/dmail_test.rb

@@ -14,10 +14,11 @@ class DmailTest < ActiveSupport::TestCase
 
     context "that is spam" do
       should "be automatically reported and deleted" do
+        skip "Rakismet credentials not configured" unless SpamDetector.enabled?
+
         @recipient = create(:user)
         @spammer = create(:user, created_at: 2.weeks.ago, email_address: build(:email_address, address: "akismet-guaranteed-spam@example.com"))
 
-        SpamDetector.stubs(:enabled?).returns(true)
         dmail = create(:dmail, owner: @recipient, from: @spammer, to: @recipient, creator_ip_addr: "127.0.0.1")
 
         assert_equal(1, dmail.moderation_reports.count)