Add idlelib.config.IdleConf unittest.

After this unittest is merged, I would like to first cleanup config.py source code, then modify it more general way.

Yes, go ahead next with name changes and use of modern features such as sets and comprehensions.  Please discuss 'general changes before coding.  When we added Linux Modern keys a year ago, I combined two functions into IdleConf.current_colors_and_keys.  I suspect that there are other duplications to remove.  

Except for extensions, current values in default configs should not change, as doing so will break past versions.  So using them in tests is good, as it will detect unwanted changes to those files.  I hope the list of extensions goes down, but when it does, we can change the tests.

Whatever is merged with PR_2691, further improvements can be made in further PRs either on this issue or others that work on config code.  

I still think this test is slower than it should be -- the change to deep copy either made no difference or made it slower, but I will try profiling before experimenting more.

In config, the warning prints in the directory search should use _warn, which can be blocked and mocked while testing.

4% uncovered in the file is probably 6% of IdleConf.  At least some is error branches that might be interesting to trigger.

New changeset f776eb0f0e046f2fa3a96540bb42d8cf970f6c55 by terryjreedy (Louie Lu) in branch 'master':
bpo-30917: IDLE: Add config.IdleConf unittests (#2691)
https://github.com/python/cpython/commit/f776eb0f0e046f2fa3a96540bb42d8cf970f6c55

The deepcopy does not work on 3.6.  Any fix should be applied to 3.7 as well.  If nothing else, we should try to reload with read_string.

This is a blocker for merging anything else into config and test_config.

PR 2754 changed it to use `read_string`.

Thanks.  I will look at it tomorrow and if it seems ok, prepare a new combined backport and see if it passes.

Please see bpo-30968: "test_get_font (idlelib.idle_test.test_config.IdleConfTest) failure on x86 Windows7 3.x".

New changeset ed014f7e135fe021837208960237d6c37afde5be by terryjreedy (Louie Lu) in branch 'master':
bpo-30917: IDLE: Fix mock_config deepcopy to read_string (#2754)
https://github.com/python/cpython/commit/ed014f7e135fe021837208960237d6c37afde5be

New changeset 65c24c846797b93d7adb72fc400f95a570f43fa9 by terryjreedy in branch '3.6':
[3.6] bpo-30917: IDLE: Add config.IdleConf unittests (GH-2691) (#2753)
https://github.com/python/cpython/commit/65c24c846797b93d7adb72fc400f95a570f43fa9

PR2769 is the fix for master for test_get_font for #30968.  The listing here is a cross-reference.  The fix was incorporated into the 3.6 backport PR2753 before it was merged.

This issue is closed, having increased coverage from 46% to 96%.  Further work on config and test config should be new issues building on top of this one.