test_signal.rb: test iff SIGQUIT is supported

* test/ruby/test_signal.rb (test_trap_system_default): test only
  if SIGQUIT is supported, which is platform dependent.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45897 b2dd03c8-39d4-4d8f-98ff-823fe69b080e