tests: add support for ospf instances with unified configs (backport #17331)

[mergify]

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This is an automatic backport of pull request #17331 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 92ecd0a has failed:

On branch mergify/bp/stable/10.2/pr-17331
Your branch is up to date with 'origin/stable/10.2'.

You are currently cherry-picking commit 92ecd0a5d4.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   tests/topotests/lib/topogen.py
	renamed:    tests/topotests/ospf_instance_redistribute/r1/zebra.conf -> tests/topotests/ospf_instance_redistribute/r1/frr.conf
	deleted:    tests/topotests/ospf_instance_redistribute/r1/ospfd-3.conf
	modified:   tests/topotests/ospf_instance_redistribute/test_ospf_instance_redistribute.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tests/topotests/lib/topotest.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally