tests/test_state_fixtures(): bump fixtures and mark new test xfail.

The upstream generated test is not sufficiently specific, and it's hard
to determine which of the two implementations is incorrect.

The principal author of the test case (Yoichi Hirai, github @pirapira)
seems currently unavailable, so it's difficult for me to get specific
details.

Author: veox

fixtures

@@ -133,6 +133,19 @@ def expand_fixtures_forks(all_fixtures):
 }
 
 
+# These are tests that are thought to be incorrect or buggy upstream,
+# at the commit currently checked out in submodule `fixtures`.
+# Ideally, this list should be empty.
+# WHEN ADDING ENTRIES, ALWAYS PROVIDE AN EXPLANATION!
+INCORRECT_UPSTREAM_TESTS = {
+    # Upstream seems to specify that the precompile call fails, but `py-evm`
+    # handles it just fine.
+    # * https://github.com/ethereum/py-evm/pull/1224#issuecomment-417351843
+    # * https://github.com/ethereum/tests/pull/405#issuecomment-417855812
+    ('stReturnDataTest/modexp_modsize0_returndatasize.json', 'modexp_modsize0_returndatasize', 'Byzantium', 4),  # noqa: E501
+}
+
+
 def mark_statetest_fixtures(fixture_path, fixture_key, fixture_fork, fixture_index):
     fixture_id = (fixture_path, fixture_key, fixture_fork, fixture_index)
     if fixture_path.startswith('stTransactionTest/zeroSigTransa'):
@@ -144,6 +157,8 @@ def mark_statetest_fixtures(fixture_path, fixture_key, fixture_fork, fixture_ind
             return pytest.mark.skip("Skipping slow test")
     elif fixture_path.startswith('stQuadraticComplexityTest'):
         return pytest.mark.skip("Skipping slow test")
+    elif fixture_id in INCORRECT_UPSTREAM_TESTS:
+        return pytest.mark.xfail(reason="Listed in INCORRECT_UPSTREAM_TESTS.")
 
 
 def pytest_generate_tests(metafunc):