refinfra: rdv3 to use tf-a-tests repo from reference-design gitlab space

Point the tf-a-tests to the repo in infra-solutions/reference-design
gitlab space. This will allow the repo to carry patches specific to
Neoverse Reference Design project.

Signed-off-by: Raghuram Thammiraju <raghuram.thammiraju@arm.com>
Change-Id: I7fd926168dc794a8026e8681154dbb2f3f654772