context: attempt to deflake TestLayersTimeout with timer padding.

Fixes #11512.

Change-Id: Iaf98ec25fb16a2409c1ff7d110afa6477267d3e1
Reviewed-on: https://go-review.googlesource.com/13643
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/context/context_test.go b/context/context_test.go
index e64afa6..05345fc 100644
--- a/context/context_test.go
+++ b/context/context_test.go
@@ -536,7 +536,7 @@
 	if testTimeout {
 		select {
 		case <-ctx.Done():
-		case <-time.After(timeout + timeout/10):
+		case <-time.After(timeout + 100*time.Millisecond):
 			errorf("ctx should have timed out")
 		}
 		checkValues("after timeout")