RT #121230, tests for process group kill on Win32

Add tests for 111f73b5d79, the fix for kill -SIG on win32, which was
broken in 5.18.0

(A follow-up commit will clean this code up a bit)
tidy up kill0.t and kill0_child

The previous commit added some tests to kill0.t, and added the auxiliary
file kill0_child. Tidy up the new code to better match normal standards.
In particular, improve the format, grammar and clarity of the comments,
and replace q|...| with "..." where appropriate.
Also, make the temporary filename a variable, and prefix it with "tmp-",
so that if gets left around for any reason, it's more obvious that it's
just an extraneous temporary file.

(I haven't actually tested this commit on win32)