monitor_task_group() fixture can get stuck in an infinite wait

[ggainey]

Version
main

Describe the bug
If a task-group fails catastrophically before it can dispatch any tasks, then the monitor_task_group() will enter the "wait for completion" loop and never come out. task_group.all_tasks_dispatched starts as False, and is never set to True, so this check:

https://github.com/pulp/pulpcore/blob/main/pulpcore/tests/functional/__init__.py#L817

is always True.

To Reproduce
Steps to reproduce the behavior:

This PR pulp/pulp_file#969 puts the lowerbounds CI test into an infinite wait, because the fix it's testing for isn't backported to core/3.23, and fails as described above, which causes the loop to spin forever, which causes lowerbounds to never complete.

Expected behavior
monitor_task_group() needs to recognize catastrophic-failure a little better.