testing: print late arriving log line in panic

When you log after a test has completed,
the testing package panics.

Print the logged line as part of that panic,
to aid in debugging.

Change-Id: I3d6689d1eed57c03e300afe37db0c15b2f4acda4
Reviewed-on: https://go-review.googlesource.com/c/go/+/283972
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/sub_test.go

@@ -822,7 +822,7 @@ func TestLogAfterComplete(t *T) {
 						c2 <- fmt.Sprintf("subtest panic with unexpected value %v", p)
 						return
 					}
-					const want = "Log in goroutine after TestLateLog has completed"
+					const want = "Log in goroutine after TestLateLog has completed: log after test"
 					if !strings.Contains(s, want) {
 						c2 <- fmt.Sprintf("subtest panic %q does not contain %q", s, want)
 					}

src/testing/testing.go

@@ -762,7 +762,7 @@ func (c *common) logDepth(s string, depth int) {
 				return
 			}
 		}
-		panic("Log in goroutine after " + c.name + " has completed")
+		panic("Log in goroutine after " + c.name + " has completed: " + s)
 	} else {
 		if c.chatty != nil {
 			if c.bench {