ktesting: log warning on timeout

How exactly a test reacts when its context times out is unclear. In the case of
scheduler_perf, the apiserver started to shut down and the test failure then
was about not being able to reach the apiserver, which was a bit confusing.

To make it more obvious why the shutdown starts, a WARNING message gets added
to the test output by ktesting before cancellation and thus before any other
output related to that cancellation.

Author: pohly

test/utils/ktesting/contexthelper.go

@@ -59,6 +59,14 @@ func withTimeout(ctx context.Context, tb TB, timeout time.Duration, timeoutCause
 			// No need to set a cause here. The cause or error of
 			// the parent context will be used.
 		case <-after.C:
+			// Code using this tCtx may or may not log the
+			// information above when it runs into the
+			// cancellation. It's better if we do it, just to be on
+			// the safe side.
+			//
+			// Would be nice to log this with the source code location
+			// of our caller, but testing.Logf does not support that.
+			tb.Logf("\nWARNING: %s\n", timeoutCause)
 			cancel(canceledError(timeoutCause))
 		}
 	}()