selftests: forwarding: router_nh: Add a diagram

This test lacks a topology diagram, making the setup not obvious.
Add one.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/forwarding/router_nh.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+# +-------------------------+  +-------------------------+
+# | H1                      |  |                      H2 |
+# |               $h1 +     |  |               $h2 +     |
+# |      192.0.2.2/24 |     |  |   198.51.100.2/24 |     |
+# |  2001:db8:1::2/64 |     |  |  2001:db8:2::2/64 |     |
+# +-------------------|-----+  +-------------------|-----+
+#                     |                            |
+# +-------------------|----------------------------|-----+
+# | R1                |                            |     |
+# |              $rp1 +                       $rp2 +     |
+# |      192.0.2.1/24              198.51.100.1/24       |
+# |  2001:db8:1::1/64             2001:db8:2::1/64       |
+# +------------------------------------------------------+
+
 ALL_TESTS="
 	ping_ipv4
 	ping_ipv6