T4387: add more firewall checks for WLB smoketests.

[nicolas-fort]

Change Summary

Add more checks for WLB smoketest.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Add more firewall checks for WLB.

Related Task(s)

• https://phabricator.vyos.net/T4387

Component(s) name

smoketest

Proposed changes

• Add more checks for mangle table.
• Add checks for nat table.

How to test

root@vyos:/usr/libexec/vyos/tests/smoke/cli# ./test_load_balancning_wan.py 
test_check_chains (__main__.TestLoadBalancingWan) ... ok
test_table_routes (__main__.TestLoadBalancingWan) ... ok

----------------------------------------------------------------------
Ran 2 tests in 15.217s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly