tests: add writable /tmp tmpfs mount to base test configuration

tests: add writable /tmp tmpfs mount to base test configuration

This fixes CRIU checkpoint/restore failures that occur when CRIU tries
to create temporary directories for mount namespace reconstruction but
encounters a read-only filesystem.

The error was:
  Error (criu/mount.c:2955): mnt: Can't create a temporary directory: Read-only file system
  Error (criu/mount.c:3700): mnt: Can't remove the directory /tmp/.criu.mntns.Y96TTI: Device or resource busy

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>