Stop unintentionally creating a folder during tests on Windows

[rmartin16]

Changes

• Testing failure from writing the log file was actually depending on the call to mkdir() to fail.
  • This was assuming creating a root directory would fail...and it does on non-Windows systems.
  • On Windows, the call to create the root directory succeeds....but writing the logfile then fails.

Failed to save log to \a-path-that-will-cause-an-OSError...\logs\briefcase.2023_04_16-11_10_46.dev.log: [WinError 3] The system cannot find the path specified: '\\a-path-that-will-cause-an-OSError...\\logs'

• Instead of depending on actual filesystem interaction to test failure while writing the log file, this updates the test to mock the filesystem error when the file write occurs.
• This also introduces a test case for failing to create the directory....although, it's admittedly a little obtuse and probably fragile.

Reference

• Fixes Running test suite leaves a top level folder "a-path-that-will-cause-an-OSError" #1187

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Looks good; one suggested cleanup/reversion around the autouse on the now fixture.

[freakboy3742]

I'm not a big fan of autouse - outside of "whole of test suite" configuration setup, I find explicit is better than implicit. For the sake of including a now fixture in a handful of tests, which makes it clear that "this test has a specific time interpretation".

[rmartin16]

That's fair; also removed it from Windows SDK tests.

[freakboy3742]

Looks good!