As far as I understand it, Eshell only uses synchronous processes on 
MS-DOS (where 'make-process' doesn't exist). Since I don't build Emacs 
for MS-DOS, and I expect not many others do either, I'm worried that 
this support will regress. To avoid that, here are some regression tests.

It also fixes a small bug with how Eshell updated its internal markers 
when using synchronous processes. This mostly affects the tests, but 
could also cause unexpected behavior when using some of Eshell's 
buffer-navigation commands.