kselftest/arm64: Add a test case for SVE VL changes with SME active

We just fixed an issue where changing the SVE VL while SME was active could
result in us attempting to save the streaming mode SVE vectors without any
backing storage. Add a test case which provokes that issue, ideally we
should also verify that the contents of ZA are unaffected by any of what we
did.

Note that since we need to keep streaming mode enabled we can't use any
syscalls to trigger the issue, we have to sit in a loop in usersapce and
hope to be preempted. The chosen numbers trigger with defconfig on all the
virtual platforms for me, this won't be 100% on all systems but avoid an
overcomplicated test implementation.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230720-arm64-fix-sve-sme-vl-change-v2-2-8eea06b82d57@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>