MTR --mem doesn't check for writeable location

The test suite should check if the shared memory location is writeable before proceeding. For e.g. on Ubuntu 14.04 /run/shm is not a symlink to /dev/shm, as in later versions of Ubuntu or Debian, nor is it world writeable.

Discovered here:
http://buildbot.mariadb.org/#/builders/13/builds/62