Refactored the test files with helpers to test backend

[yangchenyun]

Refactored the test by extracting the following two methods:

func testRouteToBackend(t *testing.T, front net.Listener, back net.Listener, msg string)
func testNotRouteToBackend(t *testing.T, front net.Listener, back net.Listener, msg string) <-chan bool

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This change is 

[yangchenyun]

Ping for a look.

[yangchenyun]

ping?

[bradfitz]

The returned channel is weird.

And the helpers are both undocumented.

What's the point of this? The PR just says it's "refactored" ... but why?