Normalize CI and tox configuration with Briefcase

[freakboy3742]

What is the problem or limitation you are having?

Travertino has a tox configuration; however, for historical reasons, it uses slightly different targets to the pattern that are currently used by Briefcase (and other BeeWare projects) - most notably those related to coverage reporting.

Describe the solution you'd like

Travertino's tox configuration should provide the same interface used by other BeeWare projects. This includes:

• a tox -m test3X set of meta-targets
• Adding coverage generation targets
• Including a coverage report as part of CI tasks.

Describe alternatives you've considered

Additional context

Briefcase currently enforces a 100% code coverage requirement; It is highly unlikely that Travertino's test suite actually meets this requirement. Being able to easily report current coverage is an important first step to getting to coverage; the "fail if coverage < 100%" rule can be omitted as part of this work.

(Of course, if we're only missing a couple of lines of coverage, adding the extra test cases to get to 100% would also be appreciated)