topotests: lib, fix filter out "At line 73 in /usr/share/snmp/mibs/ietf/SNMPv2-PDU...

topotests: lib, fix filter out "At line 73 in /usr/share/snmp/mibs/ietf/SNMPv2-PDU" message

When testing SNMP service on FRR, the following error message may
appear on some distros.

> # snmpwalk -v2c -c public .1.3.6 1.1.1.1 <OID>
> Bad operator (INTEGER): At line 73 in /usr/share/snmp/mibs/ietf/SNMPv2-PDU
> [..then result ..]
>

The error message is due to the /etc/snmp/snmp.conf file. By default, this
file is used by both snmp server and client side. The net-snmp MIB parsing
routing loads MIBS, to bind oids with the naming scheme used by the MIBS.

> # cat /etc/frr/snmp.conf
> [snmp]
> mibs +ALL
>

A potential fix would consist in modifying the SNMPv2-PDU.mib file: the
problem is known on ubuntu distros, as the snmp-mibs-downloader package
has not updated the SNMPv2-PDU.mib file.

The choice is done to not modify the original distro where the test is run
on. Fix the topotests by ignoring the 'SNMPv2-PDU line 73" error message, and
keep the other error messages that may happen, for instance, when an
unknown oid name value is requested.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>