regression 1016: add a subtest for non-NULL memref of size 0

Add a subtest to assert that the implementation allows to forward
non-NULL memref from a TA to another TA.

Regression 1016 already contains tests related to forwarding memref
between TAs. Thus extend this existing test with the subtest described
above instead of writing a new one.

The Global Platform specification allows this, however, at the time of
writing, optee-os will panic. A fix is proposed at [1].

[1] core: tee_svc.c: allow to pass non-NULL memref of size 0
Link: https://github.com/OP-TEE/optee_os/pull/6405



Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>