kselftest/arm64: Validate that changing one VL type does not affect another

On a system with both SVE and SME when we change one of the VLs this should
not result in a change in the other VL. Add a check that this is in fact
the case to vec-syscfg.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230720-arm64-fix-sve-sme-vl-change-v2-3-8eea06b82d57@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>