kselftest/arm64: signal: Skip SVE signal test if not enough VLs supported

kselftest/arm64: signal: Skip SVE signal test if not enough VLs supported

On platform where SVE is supported but there are less than 2 VLs available
the signal SVE change test should be skipped instead of failing.

Reported-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220524103149.2802-1-cristian.marussi@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>