kselftest/arm64: Actually test SME vector length changes via sigreturn

The test case for SME vector length changes via sigreturn use a bit too
much cut'n'paste and only actually changed the SVE vector length in the
test itself. Andre's recent factoring out of the initialisation code caused
this to be exposed and the test to start failing. Fix the test to actually
cover the thing it's supposed to test.

Fixes: 4963aeb3 ("kselftest/arm64: signal: Add SME signal handling tests")
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/20240829-arm64-sme-signal-vl-change-test-v1-1-42d7534cb818@kernel.org


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