Set return type of base TestCase methods

From the [PHPUnit 8 release notes][1], the `TestCase` methods below now declare a `void` return type:

- `setUpBeforeClass()`
- `setUp()`
- `assertPreConditions()`
- `assertPostConditions()`
- `tearDown()`
- `tearDownAfterClass()`
- `onNotSuccessfulTest()`

[1]: https://phpunit.de/announcements/phpunit-8.html

tests/IseedTest.php

@@ -17,7 +17,7 @@ public function __construct()
         static::$testStubsDir = __DIR__.'/stubs';
     }
 
-    public function tearDown(): void
+    protected function tearDown(): void
     {
         m::close();
     }