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/user_upgrade_test.rb

@@ -44,6 +44,8 @@ class UserUpgradeTest < ActiveSupport::TestCase
       context "for a gifted upgrade" do
         context "to Gold" do
           should "prefill the Stripe checkout page with the purchaser's email address" do
+            skip "Stripe API keys not configured" unless UserUpgrade.enabled?
+
             @user = create(:user, email_address: build(:email_address))
             @user_upgrade = create(:gift_gold_upgrade, purchaser: @user)
             @checkout = @user_upgrade.create_checkout!