I reran external.bat in all three versions and compile runs better.

Test_idle with -ugui fails to find _tkinter after another test that runs tk (including itself) *in the same process*. Adding -j2 to run in separate process eliminates the problem.
F:\Python\dev\5\py35>pcbuild\python_d -m test -j2 -ugui test_tcl test_idle

So does not using test.test_support to run the tests. The following runs fine either at a command line or within Idle. (Does unittest run tests in a subprocess?)

from test import support; support.use_resources = ['gui']
import unittest
unittest.main('test.test_tcl', exit=False)
unittest.main('test.test_idle', exit=False)