Investigate tests that still require sleeps

[jhendrixMSFT]

There are some tests that require a time.Sleep() between actions. Some of these are due to legitimate races, e.g. in conn_test.go TestClose, starting and immediately stopping the connection never gives the connReader, connWriter, and mux goroutines to start. This is a corner-case, but the race is real.

This is further surfaced in NetConn, having to use a buffered channel for the reader (see code comment for full explanation).