See #2862508: Convert system functional tests to phpunit
See #2735005: Convert all Simpletest web tests to BrowserTestBase (or UnitTestBase/KernelTestBase)
Scope:
Everything in /core/modules/system/src/Tests/Install
Things taken into account:
- All tests were moved to /core/tests/Drupal/FunctionalTests/Installer where the BrowserTestBase version of InstallerTestBase already lives
- All setUp() logic was moved to prepareEnvironment() to take into account the order in which the file system is set up in BrowserTestBase