Improve adapter integration coverage

[samuel-williams-shopify]

This leans the framework adapter tests toward the way Covered is used in practice: each adapter is exercised through a real subprocess entrypoint, while the existing focused parent-process probes preserve self-coverage for lines that run before coverage starts or after it finishes.

• Assert RSpec and Minitest framework output in the existing coverage subprocess tests.
• Add a Sus subprocess fixture that runs through Covered::Sus with coverage enabled.
• Add an autostart subprocess fixture that exercises ruby -rcovered/autostart and verifies report output.
• Keep Covered self-coverage at 100%.