KVM: arm64: selftests: Add basic SError injection test

KVM: arm64: selftests: Add basic SError injection test

Add tests for SError injection considering KVM is more directly involved
in delivery:

 - Pending SErrors are taken at the first CSE after SErrors are unmasked

 - Pending SErrors aren't taken and remain pending if SErrors are masked

 - Unmasked SErrors are taken immediately when injected (implementation
   detail)

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250708172532.1699409-25-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>