Add test for diofant#12

tests/test_functions.py

@@ -1,15 +1,25 @@
 import math
 
+import pytest
 from hypothesis import given
 from hypothesis.strategies import integers
 
-from gmp import mpz, gcd, isqrt
+from gmp import mpz, factorial, gcd, isqrt
 
 
 @given(integers(min_value=0))
 def test_isqrt(x):
     mx = mpz(x)
-    assert isqrt(mx) == isqrt(x) == math.isqrt(x)
+    r = math.isqrt(x)
+    assert isqrt(mx) == isqrt(x) == r
+
+
+@pytest.mark.xfail(reason="diofant/python-gmp#12")
+@given(integers(min_value=0, max_value=12345))
+def test_factorial(x):
+    mx = mpz(x)
+    r = math.factorial(x)
+    assert factorial(mx) == factorial(x) == r
 
 
 @given(integers(), integers())