Cygwin: CI: Run cygserver for tests

Cygwin: CI: Run cygserver for tests

Note that cygserver must be run using the same cygwin1.DLL as test
programs, as they communicate over a named pipe whose name contains the
'installation key' (which is a hash of the cygwin1.dll's path).

We run cygserver via 'cmd' to avoid the special code which handles a
cygwin parent process starting a cygwin child process, which assumes the
same version of cygwin in both.