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/mastodon_test.rb

@@ -146,6 +146,8 @@ module Sources
 
     context "A deleted or invalid source" do
       setup do
+        skip "Pawoo keys not set" unless Danbooru.config.pawoo_client_id
+
         @site1 = Sources::Strategies.find("https://pawoo.net/@nantokakun/105643037682139899") # 404
         @site2 = Sources::Strategies.find("https://img.pawoo.net/media_attachments/files/001/297/997/original/c4272a09570757c2.png")
 

test/unit/sources/nico_seiga_test.rb

@@ -2,6 +2,10 @@ require 'test_helper'
 
 module Sources
   class NicoSeigaTest < ActiveSupport::TestCase
+    setup do
+      skip "NicoSeiga credentials not configured" unless Sources::Strategies::NicoSeiga.enabled?
+    end
+
     context "The source site for nico seiga" do
       setup do
         @site_1 = Sources::Strategies.find("http://lohas.nicoseiga.jp/o/910aecf08e542285862954017f8a33a8c32a8aec/1433298801/4937663")

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

test/unit/sources/pixiv_test.rb

@@ -2,6 +2,10 @@ require 'test_helper'
 
 module Sources
   class PixivTest < ActiveSupport::TestCase
+    setup do
+      skip "Pixiv credentials not configured" unless Sources::Strategies::Pixiv.enabled?
+    end
+
     def assert_illust_id(illust_id, url)
       site = Sources::Strategies.find(url)
       assert_equal(illust_id, site.illust_id)