Add test for Preact support

[kale-stew]

Description

This PR introduces tests to the Preact support that was introduced in #67.

We still need to figure out how to pass props directly to the updated Preact component being updated here to verify the container's update fn is invoked (test case no. 3)

Checklist:

• All tests are passing (Not yet!)
• Benchmark performance has not significantly decreased
• Bundle size has not been significantly impacted
• The bundle size badge has been updated to reflect the new size

Bundle size is not impacted by tests

[kale-stew]

Ok, replied to the above feedback and got the tests synced up with each other, but now we're seeing legitimate failures for actual test conditions.

Case 1 - compares without warning or errors: expected 0, got 1
Case 2 - elements of same type and props are equal: expected 1, got 2

Edit: It looks like our Preact check isn't passing the Preact elements we've created in these tests whatsoever, which is why it's always re-rendering. Going to evaluate the Preact elements themselves, I assume I've duplicated React behavior that is incompatible with PreactChild and PreactContainer.

Edit, part 2: The offending check is a.$$typeOf which only returns React element data, it's coming back as undefined for everything Preact. Going to tie it to the React validator for now, until I can get more feedback from more Preact-smart folks 👍

[kale-stew]

Our tests are now failing because @testing-library/dom is incompatible with the node v8 engine that AppVeyor runs first. 😞

The library only just recently patch released support for Node v10 (one whole version newer) 13 days ago and has stated they plan to deprecate support whenever it's officially EOL (sometime mid-2021). This will be an issue for our current CI approach.

Can we skip specific tests for a specific Node version? Should we go about these tests a different way entirely? What % of users are using preact and node v<10?

[JoviDeCroock]

Awesome work 💯

[ryan-roemer]

@kale-stew -- I've added two small changes:

1. 11a60b4 - Our coverage wasn't being generated in Node.js.
2. d7f3b6f - Transpile a specific preact library. You can use this technique for other deps if we find them (if this isn't enough for ie11)

[ryan-roemer]

Updated cov to codecov (removed coveralls) with badge!

Also note, we're not running travis, but should pass because this branch doesn't target master. Will catch up in preact-support branch.

[ryan-roemer]

I'd leave this as "drop test support for Node 8". As our library works untranspiled in ie11, it definitely works in lower nodes 😉