The existing mod_tidy test uses replacements, but doesn't replace
modules that can also be resolved by fetching from GOPROXY, and
doesn't check the differences between the internal and external views.

This new test clarifies that interaction with a more realistic example.

Change-Id: I2bb2028148f4b7b95c3bfcc54b3976a49515379a
Reviewed-on: https://go-review.googlesource.com/c/140859
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>