Relational: add tests for `Eq(p, 0)` where p is positive and similar

TODO:
- [x] test code.
- [x] Currently XFAIL'd.
- [ ] there are surely many related issues about this, find some.
- [X] related to assumptions
- [ ] Perhaps it is possible to fix this with something like the `dif` code in `Expr.__lt__`.  Try doing this in `Equality.__new__` before the call to `_eval_sides`.  Notably these are exactly the sorts of failures I get in Inequalities if I drop the `dif` code, see also pr #7838.