oeqa/runtime/cases/ptest: Make output content path absolute

The output content is created in current directory, because json content
has no defined absolute path to WORKDIR as in bitbake.

Signed-off-by: Andrej Valek <andrej.valek@siemens.com>
Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>