Define void return type for setUp() and tearDown() methods

As per #3182103 issue on Drupal.org, core's internal setUp() and tearDown() methods of all ……TestBase classes should be compatible with PHPUnit 8 changes.

Therefore DTT could also define "void" return type in its ExistingSiteBase::setUp() and ExistingSiteBase::tearDown() methods.

However, the package's required minimum PHP version constraint currently is locked at >=7.0.8 (last touched 2 years ago by Jibran), but the "void" return type was introduced in PHP 7.1.0. Therefore we should consider raising the required minimum PHP version constraint accordingly.