CUBIC app limited behavior is not fully tested

[omansfeld]

https://github.com/mozilla/neqo/blob/67ad82d7ae7a72e67034ab285854522918a8b0af/neqo-transport/src/cc/cubic.rs#L276-L280

The above function can be changed to fn on_app_limited(&mut self) {} and all tests still pass.

As per the RFC:

CUBIC does not increase its congestion window if a flow is application limited. Per Section 4.2, it is required that t in Figure 1 not include application-limited periods, such as idle periods; otherwise, Wcubic(t) might be very high after restarting from these periods.

We should assert on the reduced w_cubic(t) growth after an idle period. Or if that is not feasible at least assert on the time value after the app limited period is over.

The test in question is: https://github.com/mozilla/neqo/blob/67ad82d7ae7a72e67034ab285854522918a8b0af/neqo-transport/src/cc/classic_cc.rs#L1146-L1147

[omansfeld]

Might fix as part of #2535 but I wanted to note it down for future reference anyways.