Issue #1901670 by msonnabaum, kim.pepper, effulgentsia: Start using PHPUnit for unit tests.

...ests/Drupal/breakpoint/Tests/BreakpointMediaQueryTest.php

core/modules/simpletest/simpletest.module

core/modules/simpletest/simpletest.pages.inc

core/phpunit.xml.dist

core/scripts/run-tests.sh

...l/Tests/Core/Cache/BackendChainImplementationUnitTest.php

core/tests/Drupal/Tests/Core/Cache/NullBackendTest.php

core/tests/Drupal/Tests/Core/NestedArrayUnitTest.php

core/tests/Drupal/Tests/UnitTestCase.php

core/tests/bootstrap.php