ensure deprecation warning from assertDictContainsSubset points at actual test code (#26497)

diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index 607c7ae..61003d0 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -1146,7 +1146,8 @@ def assertDictEqual(self, d1, d2, msg=None):
     def assertDictContainsSubset(self, subset, dictionary, msg=None):
         """Checks whether dictionary is a superset of subset."""
         warnings.warn('assertDictContainsSubset is deprecated',
-                      DeprecationWarning)
+                      DeprecationWarning,
+                      stacklevel=2)
         missing = []
         mismatched = []
         for key, value in subset.items():
diff --git a/Lib/unittest/test/test_case.py b/Lib/unittest/test/test_case.py
index 3533485..442651e 100644
--- a/Lib/unittest/test/test_case.py
+++ b/Lib/unittest/test/test_case.py
@@ -706,6 +706,10 @@ def testAssertDictContainsSubset(self):
             with self.assertRaises(self.failureException):
                 self.assertDictContainsSubset({'foo': one}, {'foo': '\uFFFD'})
 
+        with self.assertWarns(DeprecationWarning) as warninfo:
+            self.assertDictContainsSubset({}, {})
+        self.assertEqual(warninfo.warnings[0].filename, __file__)
+
     def testAssertEqual(self):
         equal_pairs = [
                 ((), ()),