[Tests] Separate utilities for creation of dummy model and buffer

[Tests] Separate utilities for creation of dummy model and buffer

In order to make unit tests for several modules such as plugin-comm-ip
or app tests, it is required to create dummy models to be provided.
Fortunately, those procedure is already included in the
'dummy_inference' app test. Therefore, this patch separates those utility
functions for creation of dummy model and generic buffer from the
'dummy_inference' apptest. According to this change, dummy_inference
is also modified to use those separated functions.

Signed-off-by: Wook Song <wook16.song@samsung.com>