T5169: Add smoketest for CGNAT

[sever-sever]

Change Summary

Add smoketest for CGNAT

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): Smoketest

Related Task(s)

• https://vyos.dev/T5169

Related PR(s)

Component(s) name

cgnat

Proposed changes

How to test

Smoketest result

vyos@r4:~$ /usr/libexec/vyos/tests/smoke/cli/test_cgnat.py
test_cgnat (__main__.TestCGNAT.test_cgnat) ... ok

----------------------------------------------------------------------
Ran 1 test in 18.632s

OK
vyos@r4:~$ 

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