tests: Make bgp_snmp_mplsl3vpn be more forgiving

tests: Make bgp_snmp_mplsl3vpn be more forgiving

I rarely get this failure:
@classname: bgp_snmp_mplsl3vpn.test_bgp_snmp_mplsvpn
@name: test_pe1_converge_evpn
@time: 44.875
@message: AssertionError: BGP SNMP does not seem to be running
assert False
 +  where False = <bound method SnmpTester.test_oid of <lib.snmptest.SnmpTester object at 0x7fa8562eb4f0>>('bgpVersion', '10')
 +    where <bound method SnmpTester.test_oid of <lib.snmptest.SnmpTester object at 0x7fa8562eb4f0>> = <lib.snmptest.SnmpTester object at 0x7fa8562eb4f0>.test_oid
        "Wait for protocol convergence"
        tgen = get_topogen()

        r1 = tgen.gears["r1"]
        r1_snmp = SnmpTester(r1, "10.1.1.1", "public", "2c")

        assertmsg = "BGP SNMP does not seem to be running"
>       assert r1_snmp.test_oid("bgpVersion", "10"), assertmsg
E       AssertionError: BGP SNMP does not seem to be running
E       assert False
E        +  where False = <bound method SnmpTester.test_oid of <lib.snmptest.SnmpTester object at 0x7fa8562eb4f0>>('bgpVersion', '10')
E        +    where <bound method SnmpTester.test_oid of <lib.snmptest.SnmpTester object at 0x7fa8562eb4f0>> = <lib.snmptest.SnmpTester object at 0x7fa8562eb4f0>.test_oid

Under heavy system load a quick test before BGP can fully come up can result in a failed
test.  Add some extra time for snmp to come up properly.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>