Verify that /dev/shm size is >= 512M, which should be enough for `make
test' needs. If the verification fails, try to enlarge it automatically.
This helps avoid docker vpp/make test crashes (seen as SIGBUS).
Change-Id: I4e23d59ff8bf8befc320fa229fb6c9bfeb809a8f
Signed-off-by: Klement Sekera <ksekera@cisco.com>