bpo-37558: Shared memory tests are failing due to double slashes (GH-14703)

bpo-37558: Shared memory tests are failing due to double slashes (GH-14703)

With the addition of shared memory into Python 3.8, we now have three tests failing on Solaris, namely `test_multiprocessing_fork`, `test_multiprocessing_forkserver` and `test_multiprocessing_spawn`. The reason seems to be incorrect name handling which results in two slashes being prepended.

https://bugs.python.org/issue37558
(cherry picked from commit 4737265622251756a9480ab84af2442b6b986850)

Co-authored-by: Jakub Kulík <Kulikjak@gmail.com>