inetmib1(3/31): Add tests for SnmpExtensionInit

[Juan Lang]

>> These tests import snmpapi, which isn't available on win95. Do we need to
>> cater for that and do LoadLibrary/GetProcAddress ?
>
> Hm.  Yeah, I suppose so.  That or duplicate the SnmpUtil functions the
> tests use, which shouldn't be that hard either.

On second thought, I don't see how the tests can succeed without
snmpapi.  In particular, SnmpExtensionQuery sure seems to call
SnmpUtilOidAppend with called with SNMP_PDU_GETNEXT, in which case the
caller should call SnmpUtilOidFree.

We could either leak memory in order to avoid depending on snmpapi,
and duplicate SnmpUtilOidNCmp and SnmpUtilOidToA in the tests, or skip
the inetmib1 tests when snmpapi isn't available.
--Juan