Test: add clickable link to re2 rubygems.org page

  - adds armchair "developer experience"

Author: olleolleolle

test/email_reply_parser_test.rb

@@ -209,7 +209,7 @@ def test_mulitple_on
   def test_pathological_emails
     t0 = Time.now
     email("pathological")
-    assert (Time.now - t0) < 1, "Took too long, upgrade to re2 gem."
+    assert (Time.now - t0) < 1, "Took too long, upgrade to re2 gem. See https://rubygems.org/gems/re2"
   end
 
   def test_doesnt_remove_signature_delimiter_in_mid_line