test_process.rb: clock_getres may not supported

* test/ruby/test_process.rb (test_clock_getres): ignore if
  clock_getres is no supported, e.g., on old Linux.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44698 b2dd03c8-39d4-4d8f-98ff-823fe69b080e