Force t/io/openpid.t to use the alarm() watchdog strategy.

Force t/io/openpid.t to use the alarm() watchdog strategy.

* The default watchdog strategy on Win32 doesn't work because the watchdog
is executed via subshell ("cmd /c perl -e '...'"). When the test finishes
and tries to kill the watchdog all it manages to do is whack cmd.exe and
the watchdog still fires, potentially killing an innocent process.

* Trying to use the fork() strategy instead causes openpid.t's test #9's
"ok" to be lost occasionally.  The message itself is generated by the 4th
child of the test and should go directly to stdout, but sometimes the
output vanishes for unexplained reasons.  This doesn't appear to happen
without the watchdog enabled.

* Using the alarm() watchdog strategy seems to not cause any badness.