Make rpc-probe a first-class testing tool

[greg-szabo]

Enhances #723 .

• Move rpc-probe under tools/rpc-probe.
• Make it create its files under rpc/tests/kvstore-fixtures.
• Create an rpc-tests/kvstore.rs test file that reads all fixtures and JSON deserializes them.
• Make the binary emit a version.json file that has the Tendermint version it ran against.
• Possibly make rpc-probe set up docker with the kvstore endpoint (a la the kvstore-test crate using cargo-make).
• Fix all the folder traversals / -> PathBuf::join for correct cross-platform exexcution.

(Possibly in a new issue: repeat steps 2-5 and create a gaiad-fixtures using the gaiad endpoint.)

Note: I've crossed off the version control file. The reason: rpc-probe doesn't know exactly what endpoint it's running against. The docker image is defined in cargo-make. We'll have to rely on that for now.