Refs #131 - Simplified screenshot logic in test/wrappers/pages.js

The whole dance with queuing screenshots (see lastScreenshot variable) was only necessary because before async/await this was easier than making sure to not call takeScreenshot() again before it completed. So I removed this workaround, and serialized the logic in getTestCases() instead which as a side effect is now also somewhat easier to read.

Kinda unrelated to that, I also turned let screenshotFolder into const SCREENSHOT_DIR which seems more appropriate.