go/doc: do not treat methods as test functions

go/doc: do not treat methods as test functions

The example code was treating a method starting with Test
as a test function when considering whether to produce
a whole-file example or not. As a method can never be
a test function, this isn't correct.

Change-Id: Idd8ec9eaf0904af076e941d7fe7d967f6b7eef78
Reviewed-on: https://go-review.googlesource.com/125257
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>