Create a single backend acceptance test suite.

[jeffmendoza]

A new integration test suite is added under internal/testing/backend to test all backends with the same set of tests. All the test scaffolding is seutp to run all the backends. Only "CertifyBad" tests have been copied in in this first commit.

Arango, keyvalue, and ent tests are all close copies of each other. A large amount of work in implementing API changes is updating the tests for each backend. Also, we wish to have a gold standard test suite so that new backends can be tested to the same standards as the existing ones. This code supports adding new backends piecemeal and skipping tests on unimplemented node types.

I've only put CertifyBad here now, so all the setup can be reviewed before I move all the types over. The plan is to remove all the integration tests from arango, keyvalue, and ent, and just use these.

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[mihaimaruseac]

This is what I wanted for the backend testing, thank you for it!

Looking good on a quick skim, but I'll do a deeper review later (OOO until Jan)

[pxp928]

This is so much better!

[mrizzi]

I agree, this is a much better, consistent and stable approach.

[mihaimaruseac]

LGTM with a minor nit (typo). Thank you very much for this, I really like it!