test: add example test cases for redundant_assignment and missing_ret…

…urn z3 options

examples/custom_checkers/z3_option/z3_e9959_redundant_assignment.py

@@ -0,0 +1,9 @@
+def func(x: int) -> int:
+    '''
+    Preconditions:
+        - x > 10
+    '''
+    a = 10
+    if x > 0:   # by function precondition, this condition is True
+        a = 20  # `a` is reassigned before first use
+    return a

examples/custom_checkers/z3_option/z3_r9711_missing_return_statement.py

@@ -0,0 +1,8 @@
+def func(x: int) -> int:
+    '''
+    Preconditions:
+        - x in [1, 2, 3, 4, 5]
+    '''
+    if x > 5:
+        return x
+    print(x)