oeqa/concurrencytest: Fix display of test stdout/stderr

If oe-selftest is run with -j, the output to stdout/stderr is being
lost at present. Capture this and display it upon test failure. We
have code that previously tried to enable this but it wasn't functioning
correctly. This should give more usable error reports on the autobuilder.

This code will mix stdout and stderr as the output is streamed from the test
server without markup. This is most in keeping with subunit/testools though
and the easiest way to handle the various challenges here as far as I can
see.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>