bpo-38377: Add support.skip_if_broken_multiprocessing_synchronize() (GH-20944) (GH...

bpo-38377: Add support.skip_if_broken_multiprocessing_synchronize() (GH-20944) (GH-20962) (GH-20966)

On Linux, skip tests using multiprocessing if the current user cannot
create a file in /dev/shm/ directory. Add the
skip_if_broken_multiprocessing_synchronize() function to the
test.support module.

(cherry picked from commit ddbeb2f3e02a510c5784ffd74c5e09e8c70b5881)
(cherry picked from commit b1e736113484c99acb57e4acb417b91a9e58e7ff)