tests: drop timecop gem.

test/unit/comment_test.rb

@@ -143,7 +143,7 @@ class CommentTest < ActiveSupport::TestCase
         Danbooru.config.stubs(:comment_threshold).returns(1)
         p = FactoryBot.create(:post)
         c1 = FactoryBot.create(:comment, :post => p)
-        Timecop.travel(2.seconds.from_now) do
+        travel(2.seconds) do
           c2 = FactoryBot.create(:comment, :post => p)
         end
         p.reload
@@ -158,7 +158,7 @@ class CommentTest < ActiveSupport::TestCase
         post.reload
         assert_equal(c1.created_at.to_s, post.last_commented_at.to_s)
 
-        Timecop.travel(2.seconds.from_now) do
+        travel(2.seconds) do
           c2 = FactoryBot.create(:comment, :post => post)
           post.reload
           assert_equal(c2.created_at.to_s, post.last_commented_at.to_s)