Refactor Key Vault tests to use common base class

[heaths]

The Key Vault live tests' base classes are mostly the same, so it'd be worth maintaining a single base class extending RecordedTestBase that had all the clean-up logic in it with a few abstract methods to delete/purge a key/secret/cert and create a client. When changes are necessary like they were recently in PR #27311 they would only need to be done in one place, typically.

[heaths]

I don't believe there's enough value to prioritize this.