tests: use Cygwin/msys PIDs for stunnel and sshd on Windows

tests: use Cygwin/msys PIDs for stunnel and sshd on Windows

Since the Windows versions of both programs would write Windows
PIDs to their pidfiles which we cannot handle, we need to use
our known perl.exe Cygwin/msys PID together with exec() in order
to tie the spawned processes to the existance of our perl.exe

The perl.exe that is executing secureserver.pl and sshserver.pl
has a Cygwin/msys PID, because it is started inside Cygwin/msys.

Related to #5188