net/http: adaptive wait time in PersistConnLeak tests
In tests TransportPersistConnLeak and TransportPersistConnLeakShortBody,
there's a fixed wait time (100ms and 400ms respectively) to allow
goroutines to exit after CloseIdleConnections is called. This
is sometimes too short on a slow host running many simultaneous
tests.
This CL replaces the fixed sleep in each test with a sequence of
shorter sleeps, testing the number of remaining goroutines until
it reaches the threshold or an overall time limit of 500ms expires.
This prevents some failures in the plan9_arm builder, while reducing
the test time on faster machines.