After officially launching the PHPUnit initiative on 2016-11-05 we made a huge amount of progress:

We tried to convert many tests by simply moving and renaming them in an automated way. This moved 30% of the tests over “without” any hassle
After that we work on conversion per module base
We identify common problems and extract them into their own issue
We added extensive BC layer to make conversions as painless as possible
Show the graph with the number of test types since 8.1.0 from the website, everybody loves a good graph
Expanded core dev team interested in conversion.
Steps still to do

This is how you can help:

Improve documentation.
Upgrade path tests conversion.
Reduce random fails in js Tests.
Remove webtest craft once conversion are done.
Update phpunit library.
Help contrib module
Better UI
Parallel runner
Deprecate the WebTest
Deprecate simpletest
Coverage reports
Future ideas / iterations

JS testing in js
JS functional testing.
JS unit testing.
JS test runner.
Webdriver
Selenium webdriver
PhantomJS webdriver
APITestBase
Headless Drupal testing.
Headless chrome driver.
Headless firefox driver.
runtest.sh to symfony console command.