skip test_localtime_zone if force_tz_test is false

For example Solaris
https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable10x/ruby-trunk/log/20180703T091803Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63839 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_time_tz.rb

@@ -109,6 +109,7 @@ class TestTimeTZ < Test::Unit::TestCase
     t = with_tz("America/Los_Angeles") {
       Time.local(2000, 1, 1)
     }
+    skip "force_tz_test is false on this environment" unless t
     z1 = t.zone
     z2 = with_tz(tz="Asia/Singapore") {
       t.localtime.zone