test_time_tz.rb: have_tz_offset?

* test/ruby/test_time_tz.rb (TestTimeTZ::Util#have_tz_offset?): append
  ?-suffix as a predicate method.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47452 b2dd03c8-39d4-4d8f-98ff-823fe69b080e