Less wrong printed circular list tail index (bug#55395)

Less wrong printed circular list tail index (bug#55395)

When printing a circular list and `print-circle` is nil, use a
somewhat more meaningful ". #N" tail index.  The previous method for
calculating that index was based on Floyd circularity detection being
used so it had been broken ever since the change to Brent's algorithm.

The new index is correct with respect to the start of the list itself
which is what it used to be before being completely broken.
It does not take into account the nesting depth of the list context.

* src/print.c (struct print_stack_entry, print_object):
Keep track of the tortoise index (which is cheap) instead of trying
to derive it from the printed element index.
* test/src/print-tests.el (print-test-rho, print-circular):
New test.