Add conditional coverage exceptions

[freakboy3742]

What is the problem or limitation you are having?

If you run the full test suite across all Python versions and platforms, you get 100% coverage.

However, if you run the tests on a single platform and Python version, there are coverage gaps.

Describe the solution you'd like

It should be possible to run coverage on a single Python version and platform, and get a coverage report that reflects all reachable code, rather than all possible code.

Briefcase has conditional coverage that does this; we should port this configuration to Toga.

Describe alternatives you've considered

Live with the status quo. It's not the end of the world; but it would be nice to know that coverage gaps are expected with a clean coverage report.

Additional context

src/core/toga/platform.py already contains a workaroud for this - both branches of a try/except are marked no-cover. Ideally, this would be a conditional coverage, rather than unconditional on both branches.