kselftest/arm64: Teach the generic signal context validation about ZT

Add ZT to the set of signal contexts that the shared code understands and
validates the form of.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-17-f2fa0aef982f@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>