Add more tests for signals

[littledan]

There are some tests in this repo alongside the polyfill implementation which get at line-by-line code coverage, but they don’t really hit all of the semantic cases that would be interesting. One possible way to improve test coverage further would be to port tests from various different signal implementations to this API (while verifying that they are valid for us).

[davidapple]

Signals for native js is an amazing idea. Solid js has taken us into a new paradigm. Keep up the good work guys.