Python: Add tests for reachability when using nonlocal.

[taus-semmle]

Variables marked as nonlocal can give rise to false negatives for reachability when they are mutated in a different scope. Thus, a variable that is set to False, and where a local function (with said variable nonlocal mutates it to be True, is considered to be False throughout, and in particular this may lead the extrator to prune the True branch of an if-statement depending on this variable.

On an internal PR, I propose that we do not prune these edges when the variable is also mentioned in a nonlocal statement. This will inevitably lead to some false negatives, and I have added tests to this effect.

I expect these tests to fail until we've merged the internal PR. (But once this is done, we can re-run the tests, and they should then go through. I have of course verified that they work locally.)

[taus-semmle]

As expected, the tests failed. For the record, here is the relevant test output:

Expected results:

Actual results:
| nonlocal.py:9:9:9:30 | Raise | Unreachable statement. |
| nonlocal.py:21:9:21:30 | Raise | Unreachable statement. |
| nonlocal.py:31:9:31:30 | Raise | Unreachable statement. |
| nonlocal.py:40:9:40:30 | Raise | Unreachable statement. |
| nonlocal.py:51:9:51:30 | Raise | Unreachable statement. |

[RasmusWL]

lgtm 👍