Adds a new job for running unit tests in GH Actions

[smoores-dev]

We weren't actually running our unit tests on PRs! Whoops! This adds a new GH Actions job that just runs our unit tests. It's in a separate job so that we can see test results regardless of other static analysis checks succeed or fail.

In addition:

When we added support for uncontrolled state (#34), we added an unconditional forceUpdate call in dispatchTransaction that was basically impossible to wrap in an act call in tests. This means that our tests were complaining about state updates that weren't wrapped in act (though they were all performing correctly, because they were all using controlled state).

This wasn't strictly necessary; the forceUpdate call is only needed when state is uncontrolled, so we don't have to run it when a state prop is passed, which all of our tests do. To quiet the test logs, this PR makes the forceUpdate call conditional on the absence of a state prop.