reset some statics after DrupalWebTestCase's where run

Once tests are done running and HTML for the results is generated, the execution of the admin/build/simpletest page (for instance, rendering of the blocks) continues with the static values produced by the tests. module_list(), module_implements(), drupal_get_schema(), for instance, are broken.

Example : when running a site with Views enabled and a 'page' view if defined, after running some tests you get the following (abridged) call sequence :
user_block()
menu_tree('navigation')
_menu_check_access($item = menu item for the 'page' views, with 'access_callback' = 'views_access')
views_access()
views_db_object::init()
drupal_get_schema('views_view') - views was not enabled during the tests, so this returns FALSE, which of course views's code doesn't expect and you get nasty PHP error messages.

Attached patch resets a few important static variables in tearDown()