Add missing tests for FlushKey, LoadKey, and SaveKey in winreg

[briancurtin]

BPO	8579
Nosy	@pfmoore, @tjguk, @briancurtin, @PCManticore, @zware, @zooba, @nanjekyejoannah
Dependencies	bpo-21518: Expose RegUnLoadKey in winreg

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/briancurtin'
closed_at = None
created_at = <Date 2010-04-30.18:02:29.898>
labels = ['type-bug', '3.9', '3.10', '3.11', 'extension-modules', 'tests', 'OS-windows']
title = 'Add missing tests for FlushKey, LoadKey, and SaveKey in winreg'
updated_at = <Date 2021-04-25.16:05:23.014>
user = 'https://github.com/briancurtin'

bugs.python.org fields:

activity = <Date 2021-04-25.16:05:23.014>
actor = 'iritkatriel'
assignee = 'brian.curtin'
closed = False
closed_date = None
closer = None
components = ['Extension Modules', 'Tests', 'Windows']
creation = <Date 2010-04-30.18:02:29.898>
creator = 'brian.curtin'
dependencies = ['21518']
files = []
hgrepos = []
issue_num = 8579
keywords = []
message_count = 4.0
messages = ['104655', '106613', '218883', '346339']
nosy_count = 7.0
nosy_names = ['paul.moore', 'tim.golden', 'brian.curtin', 'Claudiu.Popa', 'zach.ware', 'steve.dower', 'nanjekyejoannah']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue8579'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[briancurtin]

Per the comment at the top of Lib/test/test_winreg.py, FlushKey, LoadKey, and SaveKey are currently untested.

I have a minimal patch worked up. I'll expand on it and upload shortly.

[briancurtin]

LoadKey and SaveKey require special privileges which need to manually acquired, likely via ctypes. bpo-1578269 has some code which does this for os.symlink privileges and it's about to go into py3k, so I'll try to piggyback off of that.

[PCManticore]

I'm working on this issue and I have a minimal patch ready, but it depends on bpo-21518, which proposes adding UnloadKey to winreg, so that we can unload the keys mounted with LoadKey. Also, that issue adds a mechanism for privilege acquisition, which I'll utilize for this issue.

[nanjekyejoannah]

@Claudiu.Popa are you still interested in working on this, If so please open a pull request for review.