[VPP-1646] NAT44 test case asserts

[vvalderrv]

When running "make test-debug TEST=test_nat.TestNAT44" or "make test-all-debug TEST=test_nat.TestNAT44" test_nat.TestNAT44.test_vrf_mode fails. When running the test stand alone it succeeds. The test asserts on removal of fib table because of remaining record in the table. 

This happens on clearing configuration in clear_nat44 method on nat44_add_del_address_range call.

I am running it on 18.04 ubuntu.

Trace:

/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: /opt/vpp/src/vnet/fib/ip4_fib.c:184 (ip4_fib_table_destroy) assertion `0 == fib_table->ft_total_route_counts' fails
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: received signal SIGABRT, PC 0x7febd7224428
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #0 0x00007febd7be4f1e unix_signal_handler + 0x267
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #1 0x00007febd78f8390 0x7febd78f8390
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #2 0x00007febd7224428 gsignal + 0x38
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #3 0x00007febd722602a abort + 0x16a
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #4 0x00000000004078bd vhost_user_unmap_all + 0x0
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #5 0x00007febd75f78a9 debugger + 0x1c
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #6 0x00007febd75f7ce1 _clib_error + 0x2cf
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #7 0x00007febd8c38ca6 ip4_fib_table_destroy + 0x375
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #8 0x00007febd8c4e0cf fib_table_destroy + 0x82
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #9 0x00007febd8c4e2c7 fib_table_unlock + 0x82
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #10 0x00007feb92f45d8c snat_del_address + 0x4a6
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #11 0x00007feb92f61488 vl_api_nat44_add_del_address_range_t_handler + 0x181
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #12 0x00007febd92b57b4 vl_msg_api_handler_with_vm_node + 0x26d
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #13 0x00007febd92802d4 void_mem_api_handle_msg_i + 0x59
/opt/vpp/build-root/install-vpp_debug-native/vpp/bin/vpp[17158]: #14 0x00007febd9280343 vl_mem_api_handle_msg_main + 0x4d.