Don't error in Proced tests if %CPU is a NaN

Don't error in Proced tests if %CPU is a NaN

* test/lisp/proced-tests.el (proced--cpu-at-point): New function.
(proced--assert-process-valid-cpu-refinement)
(proced-refine-test, proced-refine-with-update-test): If %CPU for any
process visited is a NaN skip the test.
(proced-update-preserves-pid-at-point-test): Fix typo in comment.
(proced--assert-process-valid-cpu-refinement-explainer): Add process
attributes to the explainer along with tweaking how the process %CPU is
obtained to account for circumstances where it's not numeric (most
notably '-nan').