Add KV Testing

[Kakapio]

I had a lot of difficulty getting the tests working in this repo. I had issues with not including the tests in workers-kv as a member of the main Cargo.toml. There were also issues with the versioning of the various nested Cargo.toml files. I think it would probably be prudent to separate the workers-kv/tests into its own member. I have not yet seen rust-based tests anywhere else in this repo so I wanted to get feedback on that.

I will make a follow-up pull request for updated dependencies and separation of testing modules. I will also look into updating the tests to use Miniflare 3 instead of Miniflare 1.

[kflansburg]

There are only a handful of endpoints here, and this very closely matches the pattern used in the main test suite, would it be simpler to just move these test cases over to worker-sandbox?