Accept non-fully qualified TestCase in StructureTest

Accept non-fully qualified TestCase in StructureTest

This makes StructureTest also recognize test classes which look like

    use PHPUnit\Framework\TestCase;

    class FooTest extends TestCase {

instead of

    class FooTest extends \PHPUnit\Framework\TestCase {

This form is preferred, for instance, in Wikibase code.

Bug: T188276
Change-Id: I5bef035df33d317893ad3ba195ecb75f3b09a62f