Run structure tests on extensions

The tests in the structure PHPUnit testsuite are used to verify the
Autoloader, ResourceLoader definitions and some basic other tests.

They are not run by Jenkins for MediaWiki extensions since we invoke:

 phpunit.php --testsuite extensions

Add the structure directory to the 'extensions' testsuite.  That will
start enforcing them on all extensions, so need to be merged with care.

We will want to backport this patch on all release branches we support
and make sure they pass for all extensions/branches :-/

Bug: T78798
Change-Id: Icb88fb3f10d203cfcb61c36c9c92864b92d6739c
Merge "Run structure tests on extensions"