twister: support defining testcases in yaml file

Some complex testsuite use macros and other mean to define testcases
which makes them unparseable. This ends up in a descripency between the
testplan and the test results.

Allow adding the testcases into the yaml and use those instead of
parsing the C files.

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