chore: Ec operations audit, part 2 (#18088)

### 🧾 Audit Context

Restructure testing of the constraints for addition of points on the
Grumpkin curve.

### 🛠️ Changes Made

Refactor tests adding all cases:
- Inputs to the addition are either witness, mixed, of both constants
- Test overriding of inputs when predicate is set to witness false
- Check that the constraint fail when the result is different from the
sum of the inputs
- Check VK independence on the inputs

### ✅ Checklist

- [ ] Audited all methods of the relevant module/class
- [ ] Audited the interface of the module/class with other (relevant)
components
- [ ] Documented existing functionality and any changes made (as per
Doxygen requirements)
- [ ] Resolved and/or closed all issues/TODOs pertaining to the audited
files
- [ ] Confirmed and documented any security or other issues found (if
applicable)
- [X] Verified that tests cover all critical paths (and added tests if
necessary)
- [ ] Updated audit tracking for the files audited (check the start of
each file you audited)

### 📌 Notes for Reviewers

Author: federicobarbacovi