Add CortexM backend tests (python) to the CI

[psiddh]

Install arm dependencies

examples/arm/setup.sh --i-agree-to-the-contained-eula
source examples/arm/ethos-u-scratch/setup_path.sh

To build cortex-m test runner

backends/cortex_m/test/build_test_runner.sh

To run cortex_m tests

pytest --config-file=backends/arm/test/pytest.ini backends/cortex_m/test
backends/cortex_m/test/ops/test_add.py

(or) simply
backends/cortex_m/test