Add test for utf-8 encoding of stdout/err

[freakboy3742]

beeware/briefcase-macOS-Xcode-template#18 identified that macOS apps weren't being started with the correct system encoding. This PR adds a test case to verify this encoding.

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]

Three review notes:

1. This PR extends the test-mode PR, so it shouldn't be merged until that PR is merged.
2. We have a chicken-and-egg problem with testing. This PR requires updated platform templates to pass CI; testing those templates requires this PR to add the test. At least for now, the templates aren't running test suites, but this is something we should consider.
3. This reveals that the problem fixed by Set utf-8 mode during Python initialisation briefcase-macOS-Xcode-template#18 also exists on other platforms.

As a result of (3), the test suite isn't green; however, once the PRs have been merged (and in the case of -app templates, the binaries are rebuilt), it should be possible to re-run the failed tests and get a green status:

• Remove redundant LSEnvironment declaration. briefcase-macOS-app-template#12
• Correct locale encoding of Python interpreter. briefcase-iOS-Xcode-template#14
• Use correct encoding when initializing Python interpreter. briefcase-linux-flatpak-template#9

I've updated this templates as well to ensure that UTF-8 is being honored. These weren't causing test failures, but that appears to be cause the CI environment is using UTF-8 as the default encoding:

• Use PEP540 UTF-8 mode. briefcase-linux-appimage-template#24

The Windows and Android templates already enable UTF-8 mode.

I've added beeware/briefcase#985 to ensure that briefcase dev uses UTF-8 mode. This is required to make the test suite pass on Windows with a CP-1252 locale.