Functional tests bugcheck due to pending filtered NBLs

[mtfriesen]

We should consume microsoft/win-net-test#67 and update our functional tests to return a resource cleanup handle whenever an RX/TX filter is set on the functional MP/LWF. This wouldn't be bulletproof (ultimately the functional test drivers need to prevent infinite NBL pends via a watchdog timer) but returning an explicit cleanup handle in test code would make cleanup requirements more apparent.