test/py: optionally ignore errors from shell commands

Sometimes it's useful to run shell commands and ignore any errors. One
example might be cleanup logic; if a test-case experiences an error, the
cleanup logic might experience an error too, and we don't want that error
to mask the original error, so we want to ignore the subsequent error.

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