Update Variable and dask tests with _argmin_base, _argmax_base

The basic numpy-style argmin() and argmax() methods were renamed when
adding support for handling multiple dimensions in DataArray.argmin()
and DataArray.argmax(). Variable.argmin() and Variable.argmax() are
therefore renamed as Variable._argmin_base() and
Variable._argmax_base().

xarray/tests/test_dask.py

@@ -209,11 +209,11 @@ def test_reduce(self):
         self.assertLazyAndAllClose(u.mean(), v.mean())
         self.assertLazyAndAllClose(u.std(), v.std())
         with raise_if_dask_computes():
-            actual = v.argmax(dim="x")
-        self.assertLazyAndAllClose(u.argmax(dim="x"), actual)
+            actual = v._argmax_base(dim="x")
+        self.assertLazyAndAllClose(u._argmax_base(dim="x"), actual)
         with raise_if_dask_computes():
-            actual = v.argmin(dim="x")
-        self.assertLazyAndAllClose(u.argmin(dim="x"), actual)
+            actual = v._argmin_base(dim="x")
+        self.assertLazyAndAllClose(u._argmin_base(dim="x"), actual)
         self.assertLazyAndAllClose((u > 1).any(), (v > 1).any())
         self.assertLazyAndAllClose((u < 1).all("x"), (v < 1).all("x"))
         with raises_regex(NotImplementedError, "only works along an axis"):

xarray/tests/test_variable.py

@@ -1657,7 +1657,7 @@ def test_reduce_funcs(self):
         assert_identical(v.all(dim="x"), Variable([], False))
 
         v = Variable("t", pd.date_range("2000-01-01", periods=3))
-        assert v.argmax(skipna=True) == 2
+        assert v._argmax_base(skipna=True) == 2
 
         assert_identical(v.max(), Variable([], pd.Timestamp("2000-01-03")))