Unit tests for swift-format CLI tool and for Plugins

[natikgadzhi]

Currently, swift-format test suite covers Rules and PrettyPrint. Several components are not covered:

• generate-pipeline that builds RuleRegistry and friends
• swift-format the CLI executable target, including Configuration, Frontend, and the whole argument-parser based thing.
• Plugins.

Disclaimer: my understanding and knowledge are very high-level and basic still, I'm only diving in.

I think with some guidance, we could split these components up into smaller chunks, and each of those chunk-unit-test-issues could be a great good-first-issue for beginners.

UPD: Stumbled at this when I was looking to add tests for executable generate-pipeline target: https://forums.swift.org/t/argumentparser-and-xctest/34523

[BrianHenryIE]

I introduced a test for the plugin in #608: PluginRunTests.swift. I expect there is some Gherkin style BDD library that could be used to write the test in a nicer way, but I've never used one in Swift and I don't know the policy on introducing third party libraries to the project (I notice all dependencies currently are Apple projects).

[ahoppen]

Tracked in Apple’s issue tracker as rdar://126948360