cli: correctly parse screenEmulation boolean flags

[brendankenny]

fixes #12246

Unlike anything that looks like a number, things that look like a boolean aren't automatically parsed as one unless set specifically. Setting the type on nested properties is still kind of janky in yargs, so it's easier to handle in the coerce callback.

Also

• adds tests for parsing of all the screenEmulation properties
• adds a new option in cli-flags so that failure conditions can be tested without yargs logging the errors and calling process.exit(). We should have more CLI flag parsing tests, and this should help make that easier.