Test strategy inference with only post-conditions (#62)

This patch adds a test case which tests that the strategy is correctly
inferred if only the post-conditions are specified.

tests/strategy_inference/test_strategy_inference.py

@@ -50,7 +50,7 @@ def some_func(x) -> None:  # type: ignore
             str(type_error),
         )
 
-    def test_without_preconditions(self) -> None:
+    def test_without_contracts(self) -> None:
         def some_func(x: int) -> None:
             pass
 
@@ -59,6 +59,16 @@ def some_func(x: int) -> None:
 
         icontract_hypothesis.test_with_inferred_strategy(some_func)
 
+    def test_with_only_preconditions(self) -> None:
+        @icontract.ensure(lambda result: result > 0)
+        def some_func(x: int) -> int:
+            return 1
+
+        strategy = icontract_hypothesis.infer_strategy(some_func)
+        self.assertEqual("fixed_dictionaries({'x': integers()})", str(strategy))
+
+        icontract_hypothesis.test_with_inferred_strategy(some_func)
+
     def test_unmatched_pattern(self) -> None:
         @icontract.require(lambda x: x > 0 and x > math.sqrt(x))
         def some_func(x: float) -> None: