Move podman_extra to testsrcdir.cfg, as it also needs to be set for the whole source directory at once.
We actually didn't have a function that ensures it is the same in all testenv.cfg files, which could lead to subtle bugs such as setting it in only one of two testenv_*.cfg files in a source directory and then having testenv pick the file where it is not set and therefore not using the option when starting the podman container that is used for all testsuites. This problem is resolved with this patch.
