cmd/compile: lock in test for column numbers in unused error

cmd/compile: lock in test for column numbers in unused error

Updates #21317

@mdempsky fixed issue #21317 with CL 66810,
so lock a test in to ensure we don't regress.

The test is manual for now before test/run.go
has support for matching column numbers so do
it old school and match expected output after
an exec.

Change-Id: I6c2a66ddf04248f79d17ed7033a3280d50e41562
Reviewed-on: https://go-review.googlesource.com/76150
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>