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/functional/uploads_controller_test.rb

@@ -81,6 +81,8 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
 
       context "for a direct link twitter post" do
         setup do
+          skip "Twitter credentials not configured" unless Sources::Strategies::Twitter.enabled?
+
           @ref = "https://twitter.com/onsen_musume_jp/status/865534101918330881"
           @source = "https://pbs.twimg.com/media/DAL-ntWV0AEbhes.jpg:orig"
         end
@@ -116,6 +118,7 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
 
       context "for a pixiv post" do
         setup do
+          skip "Pixiv credentials not configured" unless Sources::Strategies::Pixiv.enabled?
           @ref = "http://www.pixiv.net/member_illust.php?mode=medium&illust_id=49270482"
           @source = "https://i.pximg.net/img-original/img/2015/03/14/17/53/32/49270482_p0.jpg"
         end
@@ -253,6 +256,8 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
 
       context "for a 2+ minute long video" do
         should "allow the upload if the user is an admin" do
+          skip "Twitter keys are not set" unless Danbooru.config.twitter_api_key
+
           @source = "https://twitter.com/7u_NABY/status/1269599527700295681"
           post_auth uploads_path, create(:admin_user, created_at: 1.week.ago), params: { upload: { tag_string: "aaa", rating: "q", source: @source }}
           assert_redirected_to Upload.last
@@ -323,8 +328,8 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         should_upload_successfully("https://nijie.info/view_popup.php?id=213043")
         should_upload_successfully("https://pic.nijie.net/03/nijie_picture/728995_20170505014820_0.jpg")
 
-        should_upload_successfully("https://pawoo.net/web/statuses/1202176")
-        should_upload_successfully("https://img.pawoo.net/media_attachments/files/000/128/953/original/4c0a06087b03343f.png")
+        should_upload_successfully("https://pawoo.net/web/statuses/1202176") if Danbooru.config.pawoo_client_id.present? # XXX
+        should_upload_successfully("https://img.pawoo.net/media_attachments/files/000/128/953/original/4c0a06087b03343f.png") if Danbooru.config.pawoo_client_id.present? # XXX
 
         should_upload_successfully("https://www.pixiv.net/en/artworks/64476642")
         should_upload_successfully("https://i.pximg.net/img-original/img/2017/08/18/00/09/21/64476642_p0.jpg")