make cargo test run all the tests correctly

[dherman]

It would be nice if people new to the codebase could run all the tests with a single, guessable command (either cargo test or cargo test --all). Right now, if you run cargo test you actually get a build error (because the tests only properly build with --features mock-network), and it won't run any tests except for the ones from the main crate.

I dug into the cargo community and found that there's a draft RFC on default features for build profiles that's been postponed for a while. So there isn't a straightforward way to set the default configuration for cargo test.

But we could perhaps hack around this by disabling all the tests by default, and having the default cargo test behavior simply be a wrapper that shells out to all the right commands with all the right configuration enabled via command-line arguments.