Equality: less automatic simplification and fix 'Eq(p, 0)' for p positive

update: no longer on top of #7959 and #7838.
Here's a case where, if only is_zero is being used, we don't need to duplicate all the testing that has taken place there. On the other hand, if someone modifies Eq in some way we want to remind them with these tests that these are cases that should be handled so these tests are guardians of that.