Add test_runner service

This is a new service with client and provider that can be
used for running tests in a secure processing environment
and retrieving the results.  The test_runner provider allows
for arbitrary test farmework backends.  The goal is to
have a cpputest backend.  In this commit, a mock backend
is included for testing the service itself.  The service
has its own access protocol defined under the protocols
top-level directory.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: If4e965c110763bd805abbdcb87e7e03cd76248b2
50 files changed