persistent-nodemap: add a way to make the picked uid predictable This will make the test less volatile and therefore easier to maintain. I am setting this is for the persistent nodemap docket first, but my target is to use similar logic for the revlogv2 (and co) test. For persisent nodemap, they are too many different in how each test variants (with rust, without rust, etc) needs new uid for this be really useful (right now). However it will be directly useful for the revlogv2 logic. Differential Revision: https://phab.mercurial-scm.org/D10753