After the fix for #20567, running test_idle and then test_ttk_guionly works, but running either after itself does not. Example command and output, which is same running with installed 3.4.0rc1 (Feb 11) or fresh debug build.

F:\Python\dev> 4\py34\pcbuild\python_d.exe -m test -ugui test_ttk_guionly test_ttk_guionly
[1/2] test_ttk_guionly
Warning -- os.environ was modified by test_ttk_guionly
[2/2] test_ttk_guionly
test test_ttk_guionly crashed -- Traceback (most recent call last):
  File "F:\Python\dev\4\py34\lib\test\test_ttk_guionly.py", line 35, in test_main
    *runtktests.get_tests(text=False, packages=['test_ttk']))
  File "F:\Python\dev\4\py34\lib\test\support\__init__.py", line 1719, in run_unittest
    _run_suite(suite)
  File "F:\Python\dev\4\py34\lib\test\support\__init__.py", line 1694, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: multiple errors occurred; run in verbose mode for details

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "F:\Python\dev\4\py34\lib\test\regrtest.py", line 1278, in runtest_inner
    test_runner()
  File "F:\Python\dev\4\py34\lib\test\test_ttk_guionly.py", line 37, in test_main
    get_tk_root().destroy()
  File "F:\Python\dev\4\py34\lib\tkinter\test\support.py", line 60, in get_tk_root
    root = tkinter.Tk()
  File "F:\Python\dev\4\py34\lib\tkinter\__init__.py", line 1807, in __init__
    self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
_tkinter.TclError: Can't find a usable init.tcl in the following directories:
    F:/Python/dev/4/py34/lib/tcl8.6 F:/Python/dev/4/py34/lib/tcl8.6 F:/Python/dev/4/lib/tcl8.6 F:/Python/dev/4/py34/library F:/Python/dev/4/library F:/Python/dev/4/tcl8.6.1/library F:/Python/dev/tcl8.6.1/library

init.tcl *is* in 4/tcl8.6.1.0/library and 4/tcltk/tcl8.6/lib. I believe it is usually found in the latter.

This might be Windows only.