When I run the following:
python ...\Python35\Lib\tkinter\test\runtktests.py
Error:
SystemError: Parent module 'tkinter.test' not loaded, cannot perform relative import
When I add "import tkinter.test" into runtktests.py,it's working.

This is rather import machinery issue. The simple reproducer:

$ ./python -c "import importlib; importlib.import_module('.bar', 'foo')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 981, in _gcd_import
  File "<frozen importlib._bootstrap>", line 931, in _sanity_check
SystemError: Parent module 'foo' not loaded, cannot perform relative import

SystemError means programming error and shouldn't be triggered by user code.

According to Issue 18018, the SystemError is changed to ImportError in 3.6, and in Python 2 (or before 3.3) it raised ValueError.

To me, the more important question is: is running runtktests.py as a file meant to be supported, or should it be run as python -m tkinter.test.runtktests?

runtktests.py was deleted here: https://github.com/python/cpython/pull/28929