AlgebraicForm checks invariance with random matrix that can be the identity

[kliem]

sage -t --long --warn-long 62.5 --random-seed=3017 src/sage/rings/invariants/invariant_theory.py
**********************************************************************
File "src/sage/rings/invariants/invariant_theory.py", line 605, in sage.rings.invariants.invariant_theory.AlgebraicForm._check_covariant
Failed example:
    quartic._check_covariant('h_covariant', invariant=True)
Expected:
    Traceback (most recent call last):
    ...
    AssertionError: not invariant
Got:
    <BLANKLINE>
**********************************************************************
1 item had failures:
   1 of   7 in sage.rings.invariants.invariant_theory.AlgebraicForm._check_covariant
    [890 tests, 1 failure, 4.77 s]

This is because invariance is checked with a random matrix. This is not deterministic, but the doctest claims it is.

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

Component: algebra

Issue created by migration from https://trac.sagemath.org/ticket/32118

[mwageringel]

Description changed:

--- 
+++ 
@@ -18,3 +18,5 @@
 ```
 
 This is because invariance is checked with a random matrix. This is not deterministic, but the doctest claims it is.
+
+In #29979, a doctest was marked `not tested` because of this.