sanitylib: Remove hardcoded sample.yaml & testcase.yaml in TestSuite class

This fix creates class variables SAMPLE_FILENAME & TESTCASE_FILENAME
in Class TestSuite and remove the hardcoded sample.yaml & testcase.yaml
from add_testcases function. This makes testing for sanitylib script
easier so that shippable do not detect the test_data for sanitylib
as actual zephyr testcases.

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