[KeyVault] Test utilities in a separate, private package

[sadasant]

This PR is part of a set of PRs made to explore together what are the possibilities of having, and managing, common code dependencies.

In this specific PR, we'll be exploring how it would be to move the common code out of KeyVault-Keys into a separate, private, unpublished package.

Things to consider:

• I haven't dug through the setting files to make sure they're as clean as they can possibly be.
• I haven't moved out the same common test code from KeyVault-Secrets and KeyVault-Certificates to avoid having to have so many changes in one single PR.
• CI might not pass right away, but I'm confident I can get it to work once we pick an approach.

To do:

• Come up with a list of pros and cons.
• Fix CI.

Your review is appreciated.

Please also check the other PRs:

• [KeyVault] Test utilities in a separate folder, through symbolic links and scripts [KeyVault] Test utilities in a separate folder, through symbolic links and scripts #7612
• [KeyVault] Common public source code unified through a private, bundled package. [DO NOT MERGE] [KeyVault] Common public source code unified through a private, bundled package. #7613

[sadasant]

Closing on favor of #7612