Use absolute imports in IDLE tests (GH-26581)

Relative imports do not work when running test_x as main.

Author: terryjreedy

Lib/idlelib/idle_test/test_colorizer.py

@@ -3,7 +3,7 @@
 from test.support import requires
 import unittest
 from unittest import mock
-from .tkinter_testing_utils import run_in_tk_mainloop
+from idlelib.idle_test.tkinter_testing_utils import run_in_tk_mainloop
 
 from functools import partial
 import textwrap