Polynomial generic power trunk broken

[kliem]

From #29979:

Doctesting 1 file.
sage -t --long --random-seed=151058820726654196682836430928254760259 src/sage/rings/polynomial/polynomial_element.pyx
**********************************************************************
File "src/sage/rings/polynomial/polynomial_element.pyx", line 11581, in sage.rings.polynomial.polynomial_element.generic_power_trunc
Failed example:
    for S in [ZZ, GF(3)]:
        R = PolynomialRing(S, 'x')
        for _ in range(100):
            p = R.random_element()
            n = ZZ.random_element(0, 100)
            prec = ZZ.random_element(0, 100)
            assert p.power_trunc(n, prec) == generic_power_trunc(p, n, prec), "p = {} n = {} prec = {}".format(p, n, prec)
Exception raised:
    Traceback (most recent call last):
      File "/srv/public/kliem/sage/local/lib/python3.7/site-packages/sage/doctest/forker.py", line 714, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/srv/public/kliem/sage/local/lib/python3.7/site-packages/sage/doctest/forker.py", line 1133, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.rings.polynomial.polynomial_element.generic_power_trunc[1]>", line 7, in <module>
        assert p.power_trunc(n, prec) == generic_power_trunc(p, n, prec), "p = {} n = {} prec = {}".format(p, n, prec)
    AssertionError: p = 0 n = 0 prec = 93
**********************************************************************
1 item had failures:
   1 of   3 in sage.rings.polynomial.polynomial_element.generic_power_trunc
    [2631 tests, 1 failure, 8.27 s]
----------------------------------------------------------------------
sage -t --long --random-seed=151058820726654196682836430928254760259 src/sage/rings/polynomial/polynomial_element.pyx  # 1 doctest failed

In #29979, a doctest was marked not tested because of this.

Component: algebra

Issue created by migration from https://trac.sagemath.org/ticket/32075