chore: make labels tests more robust to test order

[tswast]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes internal issue b/459432106 🦕

[tswast]

This will make it easier to look through https://github.com/googleapis/python-bigquery-dataframes/actions?query=branch%3Amain+event%3Apush to see when failures started.