testing: provide additional information when test funcs panic

Flush the output log up to the root when a test panics. Prior to
this change, only the current test's output log was flushed to its
parent, resulting in no output when a subtest panics.

For the following test function:

	func Test(t *testing.T) {
		for i, test := range []int{1, 0, 2} {
			t.Run(fmt.Sprintf("%v/%v", i, test), func(t *testing.T) {
				_ = 1 / test
			})
		}
	}

Output before this change:

	panic: runtime error: integer divide by zero [recovered]
		panic: runtime error: integer divide by zero
	(stack trace follows)

Output after this change:

	--- FAIL: Test (0.00s)
	    --- FAIL: Test/1/0 (0.00s)
	panic: runtime error: integer divide by zero [recovered]
	(stack trace follows)

Fixes #32121

Change-Id: Ifee07ccc005f0493a902190a8be734943123b6b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/179599
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -860,7 +860,6 @@ func tRunner(t *T, fn func(t *T)) {
 			t.Errorf("race detected during execution of test")
 		}
 
-		t.duration += time.Since(t.start)
 		// If the test panicked, print any test output before dying.
 		err := recover()
 		signal := true
@@ -877,10 +876,20 @@ func tRunner(t *T, fn func(t *T)) {
 		}
 		if err != nil {
 			t.Fail()
-			t.report()
+			// Flush the output log up to the root before dying.
+			t.mu.Lock()
+			root := &t.common
+			for ; root.parent != nil; root = root.parent {
+				root.duration += time.Since(root.start)
+				fmt.Fprintf(root.parent.w, "--- FAIL: %s (%s)\n", root.name, fmtDuration(root.duration))
+				root.parent.mu.Lock()
+				io.Copy(root.parent.w, bytes.NewReader(root.output))
+			}
 			panic(err)
 		}
 
+		t.duration += time.Since(t.start)
+
 		if len(t.sub) > 0 {
 			// Run parallel subtests.
 			// Decrease the running count for this test.