gh-37177: `sage -t`: Fix handling of `--probe all` broken by #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: #37177
Reported by: Matthias Köppe
Reviewer(s):

Author: Release Manager

src/sage/doctest/parsing.py

@@ -877,7 +877,7 @@ class SageDocTestParser(doctest.DocTestParser):
     optional_tags: Union[bool, set[str]]
     optional_only: bool
     optionals: dict[str, int]
-    probed_tags: set[str]
+    probed_tags: Union[bool, set[str]]
 
     def __init__(self, optional_tags=(), long=False, *, probed_tags=(), file_optional_tags=()):
         r"""
@@ -916,7 +916,10 @@ def __init__(self, optional_tags=(), long=False, *, probed_tags=(), file_optiona
                 self.optional_tags.remove('sage')
             else:
                 self.optional_only = True
-        self.probed_tags = set(probed_tags)
+        if probed_tags is True:
+            self.probed_tags = True
+        else:
+            self.probed_tags = set(probed_tags)
         self.file_optional_tags = set(file_optional_tags)
 
     def __eq__(self, other):