win32: add pipe-io-concurrent

Implement test case suggested by Ryan Lortie on bug:
https://bugzilla.gnome.org/show_bug.cgi?id=679288

"There is a potential race here that's really unlikely to happen, but
here we go: We are trying to read from the same socket in two threads.
Some data comes. That causes the poll() in both threads (above) to
finish running. Then the cancellable is checked above. We now find
ourselves here. Only one thread will read the data. The other will
block on this function. Then the user may cancel the cancellable while
we are blocked here, but we will stay blocked...."