[perl #42161] [BUG] Parrot::Test throws "Can't spawn" warning on windows if a system call returns a negative value

# New Ticket Created by Jerry Gay
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Parrot::Test throws a "Can't spawn" warning on windows if a system call
returns a negative value. i have a small patch which implements "no warnings
'exec'" around the system call in 'run_command', which fixes this. i'll
commit it as soon as i have a ticket number to refer to in a source comment.

~jerry