test: tolerate ENOMEM on packet mmap creation.

It is not possible to determine beforehand if the packet mmap allocation
will succeed or not. However, even on memory shortage, if must return
the proper error and keep running.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
test: at least one packet mmap allocation must work.

The smallest allocation allocates 8 frames, 2kB each. Even though some
systems might be memory constrained, if this cannot be achieved,
dabba will never run properly on it.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>