tests/tcg: split run-test into user and system variants

tests/tcg: split run-test into user and system variants

We can't rely on shell redirect magic to get things right so lets
setup a common output chardev that is expecting to write to files. As
we have split run-test up we might as well move the default monitor
bits into the call.

Finally a little make sophistry is required to correctly quote
$(COMMA) and as we don't inherit common rules we have our own little
copy here.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>