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>

src/net/dnsclient_unix_test.go

@@ -784,7 +784,7 @@ func TestRetryTimeout(t *testing.T) {
 			return nil, poll.ErrTimeout
 		}
 
-		if deadline == deadline0 {
+		if deadline.Equal(deadline0) {
 			t.Error("deadline didn't change")
 		}