Fix the usage of command line arguments for reference_model

- Users have to specify the --test_desc filename argument for using
  reference_model
- Update the initTestDesc function to allow users to use pure
  command line arguments to specify all test descriptions

Signed-off-by: Jerry Ge <jerry.ge@arm.com>
Change-Id: Ia23d4ba550aace6c3cd202e21bc8fbe6e0be1cf0