test_unicode_normalize.rb: show error messages

* test/test_unicode_normalize.rb (generate_test_normalize): defer
  building explicit error messages until assertion failed.  this
  is 3% slower than @@debug = false.

  (generate_test_check_true): ditto.
  (generate_test_check_false): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48113 b2dd03c8-39d4-4d8f-98ff-823fe69b080e