qa/workunits/rest/test.py: fix pg stat test

Back in e27b0d9e42b7ac3de896c3398689bbdbe03f1bbb we changed pg stat to
not dump everything when a formatter is used.

Fixes: #10326
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #3208 from ceph/wip-10326

qa/workunits/rest/test.py: fix pg stat test