test_process.rb: fix for 32bit platforms

* test/ruby/test_process.rb (test_clock_gettime_unit): results can
  exceed Fixnum limit, especially on 32bit platforms.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44245 b2dd03c8-39d4-4d8f-98ff-823fe69b080e