context: add test for WithDeadline in the past

Adds a test case for calling context.WithDeadline() where the deadline
exists in the past.  This change increases the code coverage of the
context package.

Change-Id: Ib486bf6157e779fafd9dab2b7364cdb5a06be36e
Reviewed-on: https://go-review.googlesource.com/25007Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Sameer Ajmani <sameer@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/context/context_test.go

@@ -255,6 +255,12 @@ func TestDeadline(t *testing.T) {
 	o = otherContext{c}
 	c, _ = WithDeadline(o, time.Now().Add(4*time.Second))
 	testDeadline(c, "WithDeadline+otherContext+WithDeadline", 2*time.Second, t)
+
+	c, _ = WithDeadline(Background(), time.Now().Add(-time.Millisecond))
+	testDeadline(c, "WithDeadline+inthepast", time.Second, t)
+
+	c, _ = WithDeadline(Background(), time.Now())
+	testDeadline(c, "WithDeadline+now", time.Second, t)
 }
 
 func TestTimeout(t *testing.T) {