Issue #3564158: Allow commerce to set OrderItem price, unless it is being overridden.

Add price tests

Present test checks overridden price works. Adds a test, that with present code should also pass, which checks the price is the the purchasable entity price. Adds another test, for the issue so with the present code will fail, with a resolver supplied price.

Only override the price if set from form/handler

Changes handler only to set price if either currency or/and amount are being overridden.

Closes #3564158