MAINT: clarify the data consistency test

@michael.williams pointed out that the data consistency test is pretty unclear. It looks like there's an argument we could be using instead of the combined statement.

I also added a test for the case where one result has a nan log noise evidence but the others don't as that is a case that should fail.

test/core/result_test.py

@@ -635,7 +635,7 @@ class TestResultListError(unittest.TestCase):
         with self.assertRaises(bilby.result.ResultListError):
             self.nested_results.combine()
 
-    def test_combine_inconsistent_data_nan(self):
+     def test_combine_data_all_nan_consistent(self):
         self.nested_results[0].log_noise_evidence = np.nan
         self.nested_results[1].log_noise_evidence = np.nan
         self.nested_results.combine()