runtest(): I don't know why we don't just use TESTFN, but if we have to

runtest():  I don't know why we don't just use TESTFN, but if we have to
do bizarre things to get a temp file, I changed it to use mkstemp instead
of NamedTemporaryFile.  This tried to leave the file open while passing
its name to execfile().  On Win2K (but not Win9X), though, a file created
with O_TEMPORARY cannot be opened again, so the test failed with a
permission error when execfile tried to open it.  Closer to the truth:
a file created with O_TEMPORARY can be opened again, but only if the
file is also created with SHARE_DELETE access via the Win32 CreateFile()
function.  There's no way to get at that from MS's version of libc, though
(we'd have to ditch the "std" C file functions in favor of Win32 API
calls).