tests: Add test for semantics of for-loop that optimisation can break.

tests/basics/for3.py

+# test assigning to iterator within the loop
+for i in range(2):
+    print(i)
+    i = 2
+
+# test assigning to range parameter within the loop
+# (since we optimise for loops, this needs checking, currently it fails)
+#n = 2
+#for i in range(n):
+#    print(i)
+#    n = 0