runtests.pl/stopserver: space separate pids

The stopserver function would append pids to kill and could append them
without separating them with space properly. The result would be a very
large number that by (some implementations of) kill would be interpreted
as a negative number and that process group would be wiped...

Bug: http://curl.haxx.se/bug/view.cgi?id=3188836
Reported by: Greg Pratt