gh-37177: `sage -t`: Fix handling of `--probe all` broken by #36989
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->
<!-- Why is this change required? What problem does it solve? -->
Broken in 10.3.beta6:
```
./sage -tp --probe all src/sage/doctest/util.py
...
File "/Users/mkoeppe/s/sage/sage-rebasing/worktree-
pristine/src/sage/doctest/parsing.py", line 919, in __init__
self.probed_tags = set(probed_tags)
^^^^^^^^^^^^^^^^
TypeError: 'bool' object is not iterable
----------------------------------------------------------------------
sage -t --random-seed=131361480585541724191127226366213696278
src/sage/doctest/util.py # TypeError in doctesting framework
```
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/37177
Reported by: Matthias Köppe
Reviewer(s):
