selftests: forwarding: router_mpath_nh: Add a diagram

This test lacks a topology diagram, making the setup not obvious.
Add one.

Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+# +-------------------------+
+# |  H1                     |
+# |               $h1 +     |
+# |      192.0.2.2/24 |     |
+# |  2001:db8:1::2/64 |     |
+# +-------------------|-----+
+#                     |
+# +-------------------|----------------------+
+# |                   |                   R1 |
+# |             $rp11 +                      |
+# |      192.0.2.1/24                        |
+# |  2001:db8:1::1/64                        |
+# |                                          |
+# |  + $rp12              + $rp13            |
+# |  | 169.254.2.12/24    | 169.254.3.13/24  |
+# |  | fe80:2::12/64      | fe80:3::13/64    |
+# +--|--------------------|------------------+
+#    |                    |
+# +--|--------------------|------------------+
+# |  + $rp22              + $rp23            |
+# |    169.254.2.22/24      169.254.3.23/24  |
+# |    fe80:2::22/64        fe80:3::23/64    |
+# |                                          |
+# |             $rp21 +                      |
+# |   198.51.100.1/24 |                      |
+# |  2001:db8:2::1/64 |                   R2 |
+# +-------------------|----------------------+
+#                     |
+# +-------------------|-----+
+# |                   |     |
+# |               $h2 +     |
+# |   198.51.100.2/24       |
+# |  2001:db8:2::2/64    H2 |
+# +-------------------------+
+
 ALL_TESTS="
 	ping_ipv4
 	ping_ipv6