[test][test_format_array] Code cleanup

beat/core/test/test_format_array.py

@@ -43,5 +43,7 @@ from . import prefix
 
 def test_array_of_arrays_does_not_validate():
     df = DataFormat(prefix, "user/array_of_array/1")  # not allowed
-    assert df.valid is False
-    assert df.errors[0].find("is not valid under any of the given schemas") != -1
+    nose.tools.assert_false(df.valid)
+    nose.tools.assert_not_equal(
+        df.errors[0].find("is not valid under any of the given schemas"), -1
+    )