Add ability to pass arguments to running apps

[freakboy3742]

What is the problem or limitation you are having?

Briefcase doesn't currently allow users to pass command line arguments to the apps being executed. This would be especially useful in test mode, as it would allow passing in specific test targets (or other testing arguments) without making source code modifications to the

Describe the solution you'd like

briefcase dev and briefcase run should support "passthrough arg" syntax. i.e.:

briefcase dev --test -- --args --for test program
briefcase run iOS  -- --args --for test program

Anything before the -- argument is consumed by Briefcase; anything after the -- is ignored by Briefcase, and passed verbatim to the running app.

Describe alternatives you've considered

1. Do nothing, and expect the user to modify their program to introduce test arguments.
2. Expose specific arguments at the briefcase level for specific purposes (e.g., a --target argument to let the user specify the test target.

Passthrough arguments will be a lot more flexible.

Additional context

When this change is made, the test harness in the briefcase template should be updated to honor command line arguments and pass them to the underlying test program.