tests/grub_cmd_cryptomount: Default TMPDIR to /tmp

tests/grub_cmd_cryptomount: Default TMPDIR to /tmp

This fixes behavior where grub_cmd_cryptomount temporary files, which are
some times not cleaned up, are left in the / directory. Set TMPDIR if your
system does not have /tmp or it can not be used for some reason.

Reported-by: Thomas Schmitt <scdbackup@gmx.net>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Tested-by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>