tests: add more verification of output to divergence tests It's not obvious what the obsmarkers should look like in these tests and adding them highlights a bug that happens when commit we create to resolve the divergence doesn't get created because it would be empty. I also added a few graphlog calls to check that the divergence-resolution commit is in the right place (has right parentage).

tests: add debugobsolete calls before many obslog calls As requested by Pierre-Yves. The main reason is to catch superfluous creation of obsmarkers, as well as any future changes to how the obsmarkers get created.

stablerangecache: sanity check subranges Try to detect invalid conditions on insert as would result in deep recursions and final aborts much later. This has been observed on two different machines and the check makes it hopefully possible to find the origin of the problem.