testing: add stacktrace to Fatalf and similar

[bep]

Currently, when you have failed test, the filename+linenumber you get is where the assertion failed. This is suboptimal in situations where you do your assertions in some common test helper, e.g. assertMyCustomFormat(...).

I have created myself a function to work around this problem (see below), but it would be great if Go could do this out of the box. It would make the test failure output a little more wordy, but it is a time saver.

func Fatalf(t testing.TB, format string, args ...interface{}) {
	trace := stackTrace()
	format = format + "\n%s"
	args = append(args, trace)
	t.Fatalf(format, args...)
}

[mvdan]

If you write helper funcs like assertMyCustomFormat, why not use t.Helper()?

[bep]

If you write helper funcs like assertMyCustomFormat, why not use t.Helper()?

Because I didn't know about it ...

Thanks!