tests: move test/helpers to test/test_helpers.

The Rails convention is for test/helpers to be used for testing the view
helpers in app/helpers. We were using it to store certain utility
methods instead. Move these to test/test_helpers so that test/helpers
can be used for its intended purpose.

test/test_helpers/download_helper.rb

@@ -0,0 +1,24 @@
+module DownloadTestHelper
+  def assert_downloaded(expected_filesize, source)
+    tempfile = Tempfile.new("danbooru-test")
+    download = Downloads::File.new(source, tempfile.path)
+
+    assert_nothing_raised(Downloads::File::Error) do
+      download.download!
+    end
+
+    assert_equal(expected_filesize, tempfile.size, "Tested source URL: #{source}")
+  end
+
+  def assert_rewritten(expected_source, test_source)
+    tempfile = Tempfile.new("danbooru-test")
+    download = Downloads::File.new(test_source, tempfile.path)
+
+    rewritten_source, _, _ = download.before_download(test_source, {})
+    assert_equal(expected_source, rewritten_source, "Tested source URL: #{test_source}")
+  end
+
+  def assert_not_rewritten(source)
+    assert_rewritten(source, source)
+  end
+end