refactor(samples): convert ava tests to mocha

[nareshqlogic]

Fixes convert all sample tests from ava to mocha (googleapis/google-cloud-node#2865) (it's a good idea to open an issue first for discussion)

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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[nareshqlogic]

Hi @JustinBeckwith ,
I added try/catch while deleting instance and database in before hook otherwise it is getting failed as it is not finding instance. Let me know your view about it.

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