tests: group structures tests in their own directory

tests: group structures tests in their own directory

The phpunit root directory has two test file:

 AutoLoaderTest.php
 StructureTest.php

The later was registered in phpunit under the `structure` test suite
while the former was not registered and hence never run (bug 47750).

This patch moves both files under the `structure` subdirectory and
change the suite to look in that directory.  That will avoid us having
to manually maintain a list of test files.

Updated the __DIR__ in StructureTest.php.

Change-Id: I419c9157f32bdf7e1ff26a42f4bb3f3922b7be37