test/py: check for bad patterns everywhere we wait

Currently, bad patterns are only honored when executing a shell command.
Other cases, such as the initial boot-up of U-Boot or when interacting
with command output rather than gathering all output prior to the shell
prompt, do not currently look for bad patterns in console output. This
patch makes sure that bad patterns are honored everywhere.

One benefit of this change is that if U-Boot sandbox fails to start up,
the error message it emits can be caught immediately, rather than relying
on a (long) timeout when waiting for the expected signon message and/or
command prompt.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>