test/py: Shorten u_boot_console

test/py: Shorten u_boot_console

This fixture name is quite long and results in lots of verbose code.
We know this is U-Boot so the 'u_boot_' part is not necessary.

But it is also a bit of a misnomer, since it provides access to all the
information available to tests. It is not just the console.

It would be too confusing to use con as it would be confused with
config and it is probably too short.

So shorten it to 'ubman'.

Signed-off-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/u-boot/CAFLszTgPa4aT_J9h9pqeTtLCVn4x2JvLWRcWRD8NaN3uoSAtyA@mail.gmail.com/