classes/oeqa: Replace subprocess.check_call() with check_output()

classes/oeqa: Replace subprocess.check_call() with check_output()

If you use subprocess.check_output() the traceback will contain the output
when the command fails which is very useful for debugging. There is no
good reason not to use this everywhere.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>