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>