sanitycheck: add testcases for add_testcases function of testsuite class

test_testsuite_class.py: Add testcases for add_testcases function
of testsuite class in sanitycheck.
test_data/testcases/tests & /samples : Testcase root directory
to add all the testcases & to test add_testcases function.
conftest.py: Module for common pytest fixtures, also used for
passing data from one testcase to another.
Note: conftest.py has a class_testsuite fixture where board_root is
defined as the directory which will be added in a separate PR.

Signed-off-by: Aastha Grover <aastha.grover@intel.com>