CppEditor/CppTools: Fix plugin tests.

Starting with commit 243a6259 EditorManager::closeEditors() does not
delete the editors anymore, but calls deleteLater(). When the call
returns, all kind of checks fail since the editor widget destructor was
not yet called.

Fixed by providing and calling Core::Tests::closeAndDeleteEditors(),
which explicitly sends QEvent::DeferredDelete to the widget.

Change-Id: I3287abbe74ccf7d7a18b997a847701c8be17cb55
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>