cmd/go: move mod_tidy.txt from testdata/mod to testdata/src

cmd/go: move mod_tidy.txt from testdata/mod to testdata/src

This file is clearly a script test, not a module definition, but it's in the
wrong directory to be run as one.

Fortunately, it passes with only minor modifications (changing “..” to “.”).

Change-Id: I66a544dfde82b8348108d2596c74e174157ae297
Reviewed-on: https://go-review.googlesource.com/127615
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>