all: use time.Time.Equal instead of == in some tests

all: use time.Time.Equal instead of == in some tests

(Found by making time.Time uncomparable and rerunning std tests
locally.)

Change-Id: I4fa6fb0ba7334965362387e2f6541c17a27ac3aa
Reviewed-on: https://go-review.googlesource.com/42616
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>