scripts: tests: sanitycheck: Add testcases for TestInstance Class

test_testinstance.py: Add testcases for check_build_or_run()
create_overlay() and calculate_sizes()
conftest.py: Module for common pytest fixtures
test_data/board_config: Adding test data for board configurations

Signed-off-by: Spoorthy Priya Yerabolu <spoorthy.priya.yerabolu@intel.com>