[3006.x] Increase coverage for textfsm module

[garethgreenaway]

What does this PR do?

Increase coverage for textfsm module

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [ ] Tests written/updated

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Yes

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