kselftest/arm64: Add a test for signal frames with ZA disabled

When ZA is disabled there should be no register data in the ZA signal
frame, add a test case which confirms that this is the case.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220829155728.854947-3-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>