Adding PyBinding tests for reference model

Introduced the following tests:
- a sanity test to verify basic model functionality(in memory).
- Tests for DType variants to validate behaviour for different data types.
- Unit tests generated using tosa_verif_build_tests.

Also adding selective test execution for unit tests via pytest markers ('pyb_smoke' and 'pyb_fullsuite').

Sanity and DType tests run by default.

To run unit tests, use:
  pytest -m "pyb_smoke"
  pytest -m "pyb_fullsuite"

Signed-off-by: Deeptanshu Sekhri <deeptanshu.sekhri@arm.com>
Change-Id: I7ae3c25bc76362ac0374b07ba8ef79e154c214fc