tests: Enable running parallel unit tests for Windows.

tests: Enable running parallel unit tests for Windows.

testsuite uses mkfifo in its job dispatcher that manages
parallel unit tests. MinGW does not have a mkfifo. This
results in unit tests running serially on Windows. Right
now it takes up to approximately 40 minutes to run all the
unit tests on Windows.

This commit provides a job dispatcher for MinGW that uses
temporary files instead of mkfifo to manage parallel jobs.
With this commit, on a Windows machine with 4 cores and with
8 parallel unit test sessions, it takes approximately 8
minutes to finish a unit test run.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Ben Pfaff <blp@nicira.com>