axfer: test: use memfd_create() for container-test

axfer: test: use memfd_create() for container-test

The container test program writes audio data frame to file, and read
them from the file, then validate them. For the operations, usage of
any in-memory file is good to shorten time of overall operations.

This commit uses shm via memfd_create(). As a result, overall time to
run is shorten one half of before, depending on machine environment.

I note that we can achieve the same result by using O_TMPFILE flag in
open(2) system call, however the implementation of O_TMPFILE is to add
i-node without name on underling file system, thus it has overhead
depending on implementation in each file system. On the other hand,
memfd_create() is directly relevant to shm and expected to be less
overhead.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>