classes/testsdk: print output correctly on failure

classes/testsdk: print output correctly on failure

With Python 3 we get a bytes object from the command output and not a
string, which gives some ugly formatting for error messages unless you
decode it first.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>