oeqa: target: ssh: Fail on SSH error even when errors are ignored

oeqa: target: ssh: Fail on SSH error even when errors are ignored

Most tests running SSH commands ask for no error to be raised when the
returned status is not 0. As run() will return this status, they may
later use its value to do a similar check on their own, or completely
ignore it. But most of the tests do not check if the non-zero status is
caused by a fail of the command run on the target or by a fail of SSH
itself.

This can lead to confusion when the error does not come from the command
executed on the target but from SSH itself: test might wrongfully be
marked as PASSED or might fail with incoherent errors.

As SSH errors are always reported with exit code 255, we can easily
filter these.

Modify OESSHTarget.run() behaviour so an AssertionError is raised on SSH
failures, even when ignore_status parameter is True. Still allow to
explicitly ignore this error for the rare cases where this can be
needed.

Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>