CL 122518 rolled back an earlier CL that made "go test"
start running test binaries for packages with no *_test.go files.
Add a test as another roadblock to reintroducing that behavior
in the future.

For #26462.

Change-Id: I898103064efee8d6ae65bcf74f4dffc830eae7e9
Reviewed-on: https://go-review.googlesource.com/122595
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>