Convert testbed app to support test mode

[freakboy3742]

Converts the testbed app to a stub app with a pytest test suite, able to be run with briefcase run --test.

Converts the testbed app itself into the barest stub that will actually start. It doesn't use a GUI framework - it uses bare platform APIs. The tests have all been moved to the tests folder, and converted to pytest instead of the ad-hoc testing framework.

Takes the opportunity to add pre-commit with black etc processing.

Needs the briefcase test branch (Refs beeware/briefcase#962), plus the various testcase2 template branches to work. The CI and project configurations have been modified to use those branches; those modifications will need to be removed before this is merged.

Includes a CI configuration that runs the project to confirm that it works.

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]

CI is currently failing on the Python 3.11 AppImage test; this appears to be an issue with the image that wasn't revealed with pre-release testing (if only we had a way to run app tests in CI... :-P)

[rmartin16]

FWIW, the test suite successfully completes on my machine. Also, I noticed the 3.11 support package is using Python 3.11.0b3.

[freakboy3742]

FWIW, the test suite successfully completes on my machine. Also, I noticed the 3.11 support package is using Python 3.11.0b3.

Out of interest - what OS/version combination are you running? And which OS are you seeing the 3.11.0b3 support package on?

[rmartin16]

OS Info
I'm running Pop!_OS 22.04 (an Ubuntu derivative).

✦ ❯ uname -a
Linux jupiter 6.0.6-76060006-generic #202210290932~1667401208~22.04~d2df702 SMP PREEMPT_DYNAMIC Wed N x86_64 x86_64 x86_64 GNU/Linux

~ 
✦ ❯ lsb_release -a
No LSB modules are available.
Distributor ID:	Pop
Description:	Pop!_OS 22.04 LTS
Release:	22.04
Codename:	jammy

❯ briefcase -V
0.3.1.dev1402+gcf4ccd77

I'm seeing Python version 3.11.0b3 in the Linux 3.11 support package.

[freakboy3742]

Huh... looks like I neglected to publish a 3.11.0 version when I updated all the tags for 3.11. I'll fix that on Monday (along with working out why the AppImage 3.11 build is failing when importing sre...)