split output into stdout/stderr/status tabs ?

[Jon Jagger]

At the moment on the [test] page, stdout/stderr/status are all mashed together into a single tab called output.
Create three tabs, one for each.
The tricky part of this is choosing whether to display stdout or stderr in response to the [test] buton being pressed.
We could start with showing stderr if it is non-empty, otherwise stdout.
I suspect some (maybe several) test-frameworks will not behave properly and will be doing spurious writes to stderr.
(I recall that was the case when discovering that you could not, for all test-frameworks, rely on the value of status to tell you if all the tests passed). But we can look at that if and when it happens.