time: increase test coverage for Time.Sub

Existing tests don't check overflow and underflow case for subtraction
monotonic time.

Updates #17858

Change-Id: I95311440134c92eadd7d5e409a0fc7c689e9bf41
Reviewed-on: https://go-review.googlesource.com/107056
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/internal_test.go

@@ -76,3 +76,8 @@ func CheckRuntimeTimerOverflow() {
 	// So we fall back to hope: We hope we don't hang here.
 	<-t.C
 }
+
+var (
+	MinMonoTime = Time{wall: 1 << 63, ext: -1 << 63, loc: UTC}
+	MaxMonoTime = Time{wall: 1 << 63, ext: 1<<63 - 1, loc: UTC}
+)

src/time/time_test.go

@@ -978,6 +978,8 @@ var subTests = []struct {
 	{Date(2300, 1, 1, 0, 0, 0, 0, UTC), Date(2000, 1, 1, 0, 0, 0, 0, UTC), Duration(maxDuration)},
 	{Date(2000, 1, 1, 0, 0, 0, 0, UTC), Date(2290, 1, 1, 0, 0, 0, 0, UTC), -290*365*24*Hour - 71*24*Hour},
 	{Date(2000, 1, 1, 0, 0, 0, 0, UTC), Date(2300, 1, 1, 0, 0, 0, 0, UTC), Duration(minDuration)},
+	{MinMonoTime, MaxMonoTime, minDuration},
+	{MaxMonoTime, MinMonoTime, maxDuration},
 }
 
 func TestSub(t *testing.T) {