Make QCheck2.state.res immutable

[jmid]

QCheck2 fails with compile time warnings:

File "src/core/QCheck2.ml", line 1570, characters 4-33:
1570 |     mutable res: 'a TestResult.t;
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 69 [unused-field]: mutable record field res is never mutated.

I think this was just an oversight with the refactoring in connection with QCheck2.
The PR removes the warning and the test suite still passes.

[jmid]

Rebased on main and added a CHANGELOG entry