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

tests/runtests.pl

@@ -651,6 +651,8 @@ sub stopserver {
     #
     foreach my $server (@killservers) {
         if($run{$server}) {
+            # we must prepend a space since $pidlist may already contain
+            # a pid
             $pidlist .= " $run{$server}";
             $run{$server} = 0;
         }