testing: provide additional information when test funcs panic

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>