Add CI validation of merging of common packages

[freakboy3742]

What is the problem or limitation you are having?

#1970 revealed that a very common package (setuptools) couldn't be packaged by Briefcase in a macOS.

Describe the solution you'd like

We should perform validation that common/known problematic packages can be packaged as part of a Briefcase app.

This is most risky on macOS because of the binary merging process; but it would likely be worth validating on other platforms to make sure there are no quirks (e.g., on Flatpak).

Describe alternatives you've considered

Do nothing; the status quo works, it's just not as proactive.

Additional context

We made a similar change recently (#1910) to include pyside-addons in the automation tests to ensure that PySide apps can be signed on macOS; the underlying problem is very similar here.

Came up in the context of #1972, and validating that setuptools could be packaged.