The test was checking for 1 of 2 possible error values. But based on
goroutine scheduling and the randomness of select statement receive
cases, it was possible for a 3rd type of error to be returned.
This modifies the code (not the test) to make that third type of error
actually the second type of error, which is a nicer error message.
The test is no longer flaky. The flake was very reproducible with a
5ms sleep before the select at the end of Transport.getConn.