Fix inherently failing random_expr doctest

[rws]

The docs for random_expr read:

  This function will often raise an error because it tries to create
  an erroneous expression (such as a division by zero).

It has the following doctest:

        sage: from sage.symbolic.random_tests import *
        sage: set_random_seed(53)
        sage: random_expr(50, nvars=3, coeff_generator=CDF.random_element) # random
        (v1^(0.97134084277 + 0.195868299334*I)/csc(-pi + v1^2 + v3) + sgn(1/
...

Despite having a random seed the test run changes with every new builtin function introduced in sage/functions because the global function list changes. That's why the test was marked random. The problem however is that the test can even raise an error, as the docs state above. The"random" keyword does not catch this, and it would make the test useless anyway.

Tests are meant to test the functionality of the associated code so the test and perhaps random_expr should be rewritten such that it allows a test that does not change with a changed global function list.

[rws]

The lazy_import is not even necessary for trigger.

[rws]

New commits:

​a17755c

24425: Fix inherently failing random_expr doctest

[mmezzarobba]

I don't fully understand the context, but the change looks reasonable and really unlikely to break anything—and this ticket has been languishing for months.

[rws]

Thanks. Oddly enough, #24212 depends on this.