sagemathgh-37177: `sage -t`: Fix handling of `--probe all` broken by sagemath#36989
    
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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
```

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URL: sagemath#37177
Reported by: Matthias Köppe
Reviewer(s):

sagemathgh-37177: `sage -t`: Fix handling of `--probe all` broken by sagemath#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 sagemath#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
```

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example "Fixes sagemath#12345". -->
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- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
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URL: sagemath#37177
Reported by: Matthias Köppe
Reviewer(s):