Add tests that discriminant() of number fields is Integer #24462
Description
The absolute_discriminant() of any number field should be a Sage Integer. This is currently not properly doctested, leading to a subtle failure in #24247.
CC: @JohnCremona @loefflerd @tscrim
Component: doctest coverage
Author: Frédéric Chapoton
Branch/Commit: fd5f450
Reviewer: Travis Scrimshaw
Issue created by migration from https://trac.sagemath.org/ticket/24462