kselftest/arm64: signal: Verify that signals can't change the SVE vector length

We do not support changing the SVE vector length as part of signal return,
verify that this is the case if the system supports multiple vector lengths.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210819134245.13935-5-broonie@kernel.org


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