selftests: mlxsw: Move IPv6 decap_error test to shared directory

Now that Spectrum-1 gained ip6gre support we can move the test out of
the Spectrum-2 directory.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>