Migrate AlertingTests to Swift Testing

[finestructure]

This migrates our first test class to Swift Testing:

The conversion task less than a second and required no changes at all. You simply run swift-testing-revolutionary with a test class file as input:

❯ swift-testing-revolutionary Tests/AppTests/AlertingTests.swift
Converting AlertingTests.swift
~/P/S/spi-server on issue-3655-swift-testing-part-1

[finestructure]

Not all conversions will be this simple but I've converted AnalyzerTests in the past and that's probably one of our most complicated test classes.