internal/xcontext: avoid assumptions about test timing in TestDetach

The test previously assumed that a 500ms sleep is sufficient for a
200ms timeout to trigger. On a heavily loaded CI machine, that
assumption may not always hold — but it doesn't seem necessary for the
properties under test anyway.

Fixes #61272.

Change-Id: Iaf88a39ec06d87731769acdee66847a5e9404dc0
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/517296
Reviewed-by: Michael Matloob <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
kokoro-CI: kokoro <[email protected]>

Author: Bryan C. Mills

internal/xcontext/xcontext_test.go

@@ -15,17 +15,22 @@ type ctxKey string
 var key = ctxKey("key")
 
 func TestDetach(t *testing.T) {
-	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
 	defer cancel()
+
 	ctx = context.WithValue(ctx, key, "value")
 	dctx := Detach(ctx)
 	// Detached context has the same values.
 	got, ok := dctx.Value(key).(string)
 	if !ok || got != "value" {
 		t.Errorf("Value: got (%v, %t), want 'value', true", got, ok)
 	}
-	// Detached context doesn't time out.
-	time.Sleep(500 * time.Millisecond)
+
+	// Wait for the parent context to time out, and then give it an arbitrary
+	// amount of extra time for the cancellation to propagate if it's going to.
+	<-ctx.Done()
+	time.Sleep(5 * time.Millisecond)
+
 	if err := ctx.Err(); err != context.DeadlineExceeded {
 		t.Fatalf("original context Err: got %v, want DeadlineExceeded", err)
 	}