all: call flag.Parse from TestMain only if used

all: call flag.Parse from TestMain only if used

These don't use any flags in TestMain itself, so the call is redundant
as M.Run will do it.

Change-Id: I00f2ac7f846dc2c3ad3535eb8177616b2d900149
Reviewed-on: https://go-review.googlesource.com/33275
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>