net: Fix race condition in test.

net: Fix race condition in test.

The test code used to do this:

for _, tc := range tests {
	ch <- &tc
}

Note that &tc is always the same value here.  As the value is
received from the channel, the sender can loop around and
change the contents of tc.  This means that the receiver's
value is unstable and can change while it is in use.

R=adg, r2, rsc
CC=chris, golang-dev
https://golang.org/cl/3978043