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/artist_test.rb

@@ -348,6 +348,7 @@ class ArtistTest < ActiveSupport::TestCase
       end
 
       should "find the artist" do
+        skip "Nijie credentials not configured" unless Sources::Strategies::Nijie.enabled?
         assert_artist_found("evazion", "http://nijie.info/view.php?id=218944")
         assert_artist_found("728995",  "http://nijie.info/view.php?id=213043")
       end