oeqa: sdk: Capture stderr output

oeqa: sdk: Capture stderr output

Redirect stderr to stdout when running subcommands while doing the SDK
tests. The tests will show stdout when CalledProcessError is raised,
but any output to stderr was lost.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 7cb4e9ab8c1596281060e94a216966060103956e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>