Add base test for Series[ExtensionArray].value_counts with sort=False
I had a bug in my `PeriodArray.sort_values` implementation w.r.t sorting.

Here's a generic test that would have caught it.

```diff
diff --git a/pandas/tests/extension/base/methods.py b/pandas/tests/extension/base/methods.py
index 4e7886dd2..4550817f0 100644
--- a/pandas/tests/extension/base/methods.py
+++ b/pandas/tests/extension/base/methods.py
@@ -24,6 +24,14 @@ class BaseMethodsTests(BaseExtensionTests):
 
         self.assert_series_equal(result, expected)
 
+    def test_value_counts_no_sort(self, data_for_grouping):
+        # BB..AABC -> BCCCBA
+        data = data_for_grouping.take([0, 7, 7, 7, 0, 4])
+        # B is the second most common, but should come first with sort=False
+        result = pd.Series(data).value_counts(sort=False)
+        expected = pd.Series([1, 2, 3], data_for_grouping.take([4, 0, 7]))
+        self.assert_series_equal(result, expected)
+
     def test_count(self, data_missing):
         df = pd.DataFrame({"A": data_missing})
         result = df.count(axis='columns')
```


We could add that to `base/methods.py`. It may have to be skipped for some like Categorical, since they return a CategoricalIndex, and this would build an object index.