smoketest: T6746: add substring search in getFRRconfig()

[c-po]

Change Summary

Some daemons (e.g. bgpd) have several nested substrings/sections like

router bgp 100
  address-family ipv4 unicast
  ..
  exit-address-family
exit

We can now use getFRRconfig() with the substring option to extract only

 address-family ipv4 unicast
 ..
 exit-address-family

Making config validation more granular

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/T6746

Related PR(s)

• frr: T6746: code improvements after 10.2 update #4241

Component(s) name

Smoketest infrastructure

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

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No issues in PR Title / Commit Title

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Details

CI logs

• CLI Smoketests (no interfaces) ❌ failed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed