net/http: avoid leaking writer goroutines in tests
In TestTransportPrefersResponseOverWriteError and TestMaxBytesHandler,
the server may respond to an incoming request without ever reading the
request body. The client's Do method will return as soon as the
server's response headers are read, but the Transport will remain
active until it notices that the server has closed the connection,
which may be arbitrarily later.
When the server has closed the connection, it will call the Close
method on the request body (if it has such a method). So we can use
that method to find out when the Transport is close enough to done for
the test to complete without interfering too much with other tests.