sanitycheck: Testcases for report functions of TestSuite Class

test_reporting_testsuite.py: Adding testcases for reporting
functions of Class testSuite.
test_testsuite_class.py: Testcase for add_instances function
conftest.py: Added fixture for instances_fixture

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