Skip signal test on windows

[byroot]

Windows has no SIGUSR1.

There might be another usable signal, but this is breaking ruby master so I just want a quick fix for now.

  
    1) Error:
  TestTimeout#test_timeout_in_trap_handler:
  Test::Unit::ProxyError: unsupported signal 'SIGUSR1'
      D:/a/ruby/ruby/src/test/test_timeout.rb:428:in 'Kernel#trap'
      D:/a/ruby/ruby/src/test/test_timeout.rb:428:in 'TestTimeout#test_timeout_in_trap_handler'

FYI: @eregon

[hsbt]

👍

[eregon]

Sorry about the breaking, I expected the CI of this repo to catch that. Thanks for the quick fix.