Fix flakey proced refine tests (Bug#73441)

Fix flakey proced refine tests (Bug#73441)

* test/lisp/proced-tests.el (proced-refine-test)
(proced-refine-with-update-test): Use the much simpler CPU refinement
for testing 'proced-refine'.  The previous tests made the incorrect
assumption that refining on the PID of process A only filtered the
buffer to contain process A and its children, whereas in actuality
the children of process A's children, their children, and so on will
also be shown.
(proced-update-preserves-pid-at-point-test): Mark as unstable.