Add /test

[hinto-janai]

What

• Add a top /test directory for test related crates; the crate naming pattern is cuprate-test-$NAME
• Add skeleton crates for cuprate-test-{cli,rpc}; add CONTRIBUTING.md instructions; add CI steps

Where

Directory	Crate	Notes
/test/cli	cuprate-test-cli	This is a CLI testing harness for cuprated from #469.
/test/rpc	cuprate-test-rpc	This is the new name for cuprate-rpc-compat from #422.
/test/utils	cuprate-test-utils	This is the /test-utils crate. No changes, only moved.

Requires

• ci: extend build environments #480