Having `NeotestResultCollector` (a pytest plugin) as a inner local
class would make the code a bit difficult to read due to quite much
indentation. This commit does refactoring on NeotestResultCollector
to make it a module-level class with a reference to NeotestAdapter.
This refactoring would make easier adding more pytest plugins
(e.g., debugger integration) in the future.
There should be no changes in behaviors.
