Always include at least one category in random test data (pydata#9436)

* Always include at least one category in random test data

Otherwise, creating test data can fail randomly, depending on how the
random number generator is seeded.

(Note: we would ideally avoid using a random number generator or fix the
random seed so test data is entirely deterministic, but that's a bigger
refactor...)

* use RandomState object

xarray/tests/__init__.py

@@ -324,16 +324,16 @@ def create_test_data(
         obj["var4"] = (
             "dim1",
             pd.Categorical(
-                np.random.choice(
-                    list(string.ascii_lowercase[: np.random.randint(5)]),
+                rs.choice(
+                    list(string.ascii_lowercase[: rs.randint(1, 5)]),
                     size=dim_sizes[0],
                 )
             ),
         )
     if dim_sizes == _DEFAULT_TEST_DIM_SIZES:
         numbers_values = np.array([0, 1, 2, 0, 0, 1, 1, 2, 2, 3], dtype="int64")
     else:
-        numbers_values = np.random.randint(0, 3, _dims["dim3"], dtype="int64")
+        numbers_values = rs.randint(0, 3, _dims["dim3"], dtype="int64")
     obj.coords["numbers"] = ("dim3", numbers_values)
     obj.encoding = {"foo": "bar"}
     assert_writeable(obj)