test: introduce parent test suite format

test: introduce parent test suite format

The current structure for unit testing only allows for running a
test suite with nested test cases. This means all test cases for an
autotest must be in one suite, which is not ideal.
For example, in some cases we may want to run multiple lists of test
cases that each require different setup, so should be in separate suites.

The unit test suite struct is modified to hold a pointer to a list of
sub-testsuite pointers, along with the list of testcases as before.

Signed-off-by: Ciara Power <ciara.power@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Tested-by: Ruifeng Wang <ruifeng.wang@arm.com>