selftest: net: add a test-case for encap segmentation after GRO

selftest: net: add a test-case for encap segmentation after GRO

We had a few patches in this area and no explicit coverage so far.
The test case covers the scenario addressed by the previous fix;
reusing the existing udpgro_fwd.sh script to leverage part of the
of the virtual network setup, even if such script is possibly not
a perfect fit.

Note that the mentioned script already contains several shellcheck
violation; this patch does not fix the existing code, just avoids
adding more issues in the new one.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/768ca132af81e83856e34d3105b86c37e566a7ad.1770032084.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>