Fix missing import (and missing spelling) for test mark

cspell.json

@@ -41,6 +41,7 @@
         "Cascadia",
         "clion",
         "cmdpxl",
+        "codepoints",
         "COLORTERM",
         "conhost",
         "Consola",

tests/test_behavior.py

@@ -4,6 +4,7 @@ Run with `pytest tests/test_behavior.py`, or `pytest` to run all tests.
 """
 
 from pyfakefs.fake_filesystem import FakeFilesystem
+import pytest
 from textual.events import Paste
 
 from textual_paint.char_input import CharInput