twister: add testcase class

Now we can add the TestCase class which will hold all information about
individual testcases, their status and why they are not passing, etc.

Test cases are parsed initially and testsuite is populated with the list
of testcases it contsins. This list is duplicate for each instance
(scenario) and used to capture test case data.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>