cephfs-top: enhance fstop tests to cover testing displayed data
Right now the tests are pretty rudimentary. cephfs-top is a UI tool and writing tests can be a bit hard. Due to this developers have to manually test changes (post reviews) which can be error prone and frustrating. Enhance the fstop tests to introduce hooks to operate as a terminal script that dumps (JSON) data. This mode can then be utilized by tests to validate displayed data.