smoketest: T6614: initial support for op-mode command testing (backport #3888)

[mergify]

Change Summary

We currently lack behind testing OP-mode commands with the smoketest framework.

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):

Related Task(s)

• https://vyos.dev/T6614

Component(s) name

Smoketests

Smoketest result

cpo@LR2.wue3:~$ /usr/libexec/vyos/tests/smoke/cli/test_op-mode_show.py
test_op_mode_show_version (__main__.TestOPModeShow.test_op_mode_show_version) ... ok
test_op_mode_show_vrf (__main__.TestOPModeShow.test_op_mode_show_vrf) ... ok

----------------------------------------------------------------------
Ran 2 tests in 1.469s

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

---

This is an automatic backport of pull request #3888 done by [Mergify](https://mergify.com).

[github-actions]

👍
No issues in PR Title / Commit Title

[github-actions]

❌
warning: Unused import os in smoketest/scripts/cli/test_interfaces_l2tpv3.py:17.