testscript: allow custom cmds to set stdout/stderr

[caarlos0]

when setting up custom commands, unless you use ts.Exec, you can't really set stdout/stderr.

IMHO, we could add a ts.SetStdout or ts.WriteStdout or something similar.

[mvdan]

You mean #216? :)

You might not have seen it given that it's recent and we haven't done a release since.

[caarlos0]

Oh, yes, exactly! Sorry, I searched but failed to find that PR.

Closing! Thank you!