TFTF: Use CACTUS_ECHO_CMD in direct message tests

There were two very similar tests, that were targeted to the same
purpose. This patch keeps the logic using an echo value, as it was being
used in tests involving SPs.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: Ib278d28098e60dddf807b267951f7e2b8762e9e3
4 files changed