Python: Add tests for reachability when using nonlocal.

python/ql/test/query-tests/Statements/unreachable_nonlocal/UnreachableCode.qlref

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+Statements/UnreachableCode.ql

python/ql/test/query-tests/Statements/unreachable_nonlocal/nonlocal.py

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+
+def nonlocal_fp():
+    test = False
+    def set_test():
+        nonlocal test
+        test = True
+    set_test()
+    if test:
+        raise Exception("Foo")
+
+
+# A couple of false negatives, roughly in order of complexity
+
+# test is nonlocal, but not mutated
+def nonlocal_fn1():
+    test = False
+    def set_test():
+        nonlocal test
+    set_test()
+    if test:
+        raise Exception("Foo")
+
+# test is nonlocal and mutated, but does not change truthiness
+def nonlocal_fn2():
+    test = False
+    def set_test():
+        nonlocal test
+        test = 0
+    set_test()
+    if test:
+        raise Exception("Foo")
+
+# test is nonlocal and changes truthiness, but the function is never called
+def nonlocal_fn3():
+    test = False
+    def set_test():
+        nonlocal test
+        test = True
+    if test:
+        raise Exception("Foo")
+
+# test is nonlocal and changes truthiness, but only if the given argument is true
+def nonlocal_fn4(x):
+    test = False
+    def set_test():
+        nonlocal test
+        test = True
+    if x:
+        set_test()
+    if test:
+        raise Exception("Foo")