Run setup before tests

This now allows:

```
class TestCase extends PHPUsableTest {
  public function setUp() {
    PHPUsableTest::$current_test = $this;
  }
}
class MyTest extends TestCase {
  public function tests() {
    // code here
  }
}
```

instead of:

```
class MyTest extends PHPUsableTest {
  public function tests() {
    PHPUsableTest::$current_test = $this;
    // code here
  }
}
```