all: fix vet nits

all: fix vet nits

Fixes these vet complaints:

net/error_test.go:254: unrecognized printf flag for verb 'T': '#'
os/os_test.go:1067: arg mt for printf verb %d of wrong type: time.Time
runtime/debug/garbage_test.go:83: arg dt for printf verb %d of wrong type: time.Time

Change-Id: I0e986712a4b083b75fb111e687e424d06a85a47b
Reviewed-on: https://go-review.googlesource.com/33167
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>