selftests: router_bridge_vlan: Add a diagram

Add a topology diagram to this selftest to make the configuration easier to
understand.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+# +------------------------+                           +----------------------+
+# | H1 (vrf)               |                           |             H2 (vrf) |
+# |    + $h1.555           |                           |  + $h2               |
+# |    | 192.0.2.1/28      |                           |  | 192.0.2.130/28    |
+# |    | 2001:db8:1::1/64  |                           |  | 2001:db8:2::2/64  |
+# |    |                   |                           |  |                   |
+# |    + $h1               |                           |  |                   |
+# +----|-------------------+                           +--|-------------------+
+#      |                                                  |
+# +----|--------------------------------------------------|-------------------+
+# | SW |                                                  |                   |
+# | +--|-------------------------------+                  + $swp2             |
+# | |  + $swp1                         |                    192.0.2.129/28    |
+# | |    vid 555                       |                    2001:db8:2::1/64  |
+# | |                                  |                                      |
+# | |  + BR1 (802.1q)                  |                                      |
+# | |    vid 555 pvid untagged         |                                      |
+# | |    192.0.2.2/28                  |                                      |
+# | |    2001:db8:1::2/64              |                                      |
+# | +----------------------------------+                                      |
+# +---------------------------------------------------------------------------+
+
 ALL_TESTS="
 	ping_ipv4
 	ping_ipv6