kselftest/arm64: Verify that SSVE signal context has SVE_SIG_FLAG_SM set

Streaming mode SVE signal context should have SVE_SIG_FLAG_SM set but we
were not actually validating this. Add a check.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230117-arm64-test-ssve-za-v1-1-203c00150154@kernel.org


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