Add tests for MTE support

TF-A now has support for the new Memory Tagging Extension in ARMv8.5,
see commit 91624b7fed52 ("Merge changes from topic "jc/mte_enable"
into integration").

This patch creates and enables tests to ensure that the extension is
being properly enabled and that no undesired leakages occur.

Change-Id: I7708fb23a12e2f35a97533f296aaa53db776ddd2
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>