Improve test_sentinel_import test coverage

Author: HexDecimal

src/test_typing_extensions.py

@@ -9303,10 +9303,12 @@ def test_sentinel_copy(self):
         self.assertIs(self.SENTINEL, copy.deepcopy(self.SENTINEL))
 
     def test_sentinel_import(self):
-        self.assertIs(Sentinel._import_sentinel("TestSentinels", __name__), TestSentinels)
+        self.assertIs(Sentinel("TestSentinels"), TestSentinels)
         self.assertIs(Sentinel._import_sentinel("TestSentinels.SENTINEL", __name__), TestSentinels.SENTINEL)
         self.assertIs(Sentinel._import_sentinel("nonexistent", __name__), None)
         self.assertIs(Sentinel._import_sentinel("TestSentinels.nonexistent", __name__), None)
+        self.assertIs(Sentinel._import_sentinel("nonexistent", ""), None)
+        self.assertIs(Sentinel._import_sentinel("nonexistent", "nonexistent.nonexistent.nonexistent"), None)
 
     def test_sentinel_picklable(self):
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):