selftests: arm64: Add coverage of ptrace flags for SVE VL inheritance

Add a test that covers enabling and disabling of SVE vector length
inheritance via the ptrace interface.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20211005123537.976795-1-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>