testing: can add func Output(calldepth int, s string) for testing.T

[mei-rune]

can add func Output(calldepth int, s string) or ErrorOutput(calldepth int, s string) for testing.T.

it will helpful for assert helper library.

[adg]

If you're writing an assert helper library then you can provide your own such function.

[cespare]

This might be asking for #4899.