test: Add testenv module

[ValuedMammal]

Description

This is a follow up to #5 that introduces a TestEnv struct used for integration tests and fixes a number of nitpicks that were previously left out.

Summary:

• Add simple TestEnv struct with convenient interface for improved ergonomics
• Avoid manually implementing RPC calls (e.g. generate_to_address)
• Move auth tests to client::test_auth module
• Ignore (or remove) test using potentially insecure std::env::tempdir
• Don't call node.stop(), as this is handled by the Drop implementation of Node
• Add missing tests for existing RPC methods get_block_verbose, get_block_header_verbose
• Don't print to stdout
• Have more effective test assertions. For example it's sufficient to check that the RPC method returns the expected type. Keep some basic "sanity check" assertions, but try to avoid testing functionality that is defined outside of this library (e.g. the length of a BlockHash, and Bitcoin Core itself).

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[oleonardolima]

Thanks, it's a good idea to have some sort of TestEnv for better organization and ergonomics. An initial cACK for this one, will do a final review and ACK once it's in ready to review state.

[ValuedMammal]

self-ACK f1db39a