As part of the PHPUnit initiative a considerable part of Simpletests will be converted to PHPUnit based browser tests on February 21st 2017. A backwards compatibility layer has been implemented so that many Simpletests can be converted by just using the new BrowserTestBase base class and moving the test file. There is also a script to automatically convert test files in the conversion issue.
Developers are encouraged to use BrowserTestBase instead of Simpletest as of Drupal 8.3.0, but both test systems are fully supported during the Drupal 8 release cycle. The timeline for the deprecation of Simpletest's WebTestBase is under discussion.