Instead of two parallel files that look almost identical,
mark the expected differences in the original file.

The annotations being added here keep the tests passing,
but they also make clear a number of printing or parsing
errors that were not as easily seen when the data was
split across two files.

Fix a few diagnostic problems in cmd/internal/obj as well.

A step toward #13822.

Change-Id: I997172681ea6fa7da915ff0f0ab93d2b76f8dce2
Reviewed-on: https://go-review.googlesource.com/18823
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>