Updated TestStore DSL

[stephencelis]

This PR introduces a newer, flatter way of asserting with the test store. Instead of calling store.assert(...) with a series of steps, one can now invoke store.send and store.received directly, improving how/where XCTest failures are surfaced.

[nmccann]

Very minor (and I almost managed to comment on this before it was merged), but there are a number of usages of self.scheduler that could be simplified to scheduler. I could create a PR for that, assuming there wasn't a reason for keeping them that way.

[stephencelis]

@nmccann We may be in the minority here, but we prefer to always be explicit about self even where it can be implicit. We think it makes it clearer when a value is a property vs. a local variable or constant, especially when reading outside of Xcode, like in PRs. We'd take submissions to add self. if we've missed it anywhere, though 😁

[nmccann]

Fair enough - I'll gladly admit that it does help readability in PRs. Even if you're in the minority - it's your project and you raise some compelling points, so I'm happy to defer to your preferences :) I've long ago realized there is no "correct" choice in regards to such things - internal consistency within a project is the more important thing.