Use Go 1.7 Run method of testing.T to run the table-driven tests into
separate, named subtests. The behaviour of the tests is not modified.

Change-Id: Ia88fa59a3534e79e3f0731e948b5f8a9919b339d
Reviewed-on: https://go-review.googlesource.com/84478
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>