net/http: deflake TestClientRedirectContext
net/http: deflake TestClientRedirectContext

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.

Thanks to Github user @jaredborner for debugging.

Fixes #16049

Change-Id: I0d2a036c9555a8d2618b07bab01f28558d2b0b2c
Reviewed-on: https://go-review.googlesource.com/24748
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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