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/sources/nijie_test.rb

@@ -3,6 +3,7 @@ require 'test_helper'
 module Sources
   class NijieTest < ActiveSupport::TestCase
     setup do
+      skip "Nijie credentials not configured" unless Sources::Strategies::Nijie.enabled?
       # Add a random delay to work around test failures due to rate limiting by Nijie.
       sleep (3..5).to_a.sample
     end