tests: use mocker instead of unittest.mock where possible

Had the "pleasure" of finding a mocker.patch leak into other tests and one
of the suspicions is that mixing unittest.mock and mocker might be somehow
related.

In any case, we should use mocker where possible anyway.

tests/func/test_add.py

@@ -4,7 +4,6 @@ import os
 import shutil
 import stat
 import textwrap
-from unittest.mock import call
 
 import pytest
 
@@ -631,8 +630,8 @@ def test_add_optimization_for_hardlink_on_empty_files(tmp_dir, dvc, mocker):
     stages = dvc.add(["foo", "bar", "lorem", "ipsum"])
 
     assert m.call_count == 8
-    assert m.call_args != call(tmp_dir / "foo")
-    assert m.call_args != call(tmp_dir / "bar")
+    assert m.call_args != mocker.call(tmp_dir / "foo")
+    assert m.call_args != mocker.call(tmp_dir / "bar")
 
     for stage in stages[:2]:
         # hardlinks are not created for empty files