[writeboost] Add Stack and Tests class to use backing device only for the same
tests as Writeboost

This patch also unwrap linear stacking from the backing device.
It doesn't need to use TVM to allocate backing device.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #24 from akiradeveloper/feature/add-test-backing-only

[writeboost] Add Stack and Tests class to use backing device only for the same tests as Writeboost