catch up frozen_string_literal: true.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59139 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/test/unit/assertions.rb

@@ -541,7 +541,7 @@ EOT
             sigdesc = "SIG#{signame} (#{sigdesc})"
           end
           if status.coredump?
-            sigdesc << " (core dumped)"
+            sigdesc = "#{sigdesc} (core dumped)"
           end
           full_message = ''.dup
           message = message.call if Proc === message