TST: xfail flaky OPM tests

These tests fail intermittently for a number of reasons. Sometimes
segfaulting, sometimes FileNotFound, sometimes results differ. These
issues do not appear to be a result of xtgeo but should maybe be looked
at more closely.

tests/test_opm_integration/test_gridprop_io.py

@@ -178,6 +178,7 @@ def deck_contents(self):
 opm_setups = st.builds(OpmSetup, xtgeo_grids, st.booleans(), unit_systems)
 
 
+@pytest.mark.xfail(reason="OPM flaky")
 @pytest.mark.requires_opm
 @pytest.mark.usefixtures("setup_tmpdir")
 @settings(max_examples=5)
@@ -245,6 +246,7 @@ def test_init_props_reading(case):
     assert poro.date == "20000101"
 
 
+@pytest.mark.xfail(reason="OPM flaky")
 @pytest.mark.requires_opm
 @pytest.mark.usefixtures("setup_tmpdir")
 @settings(max_examples=5)