Add `FluentAssertions.Analyzers` and fix all warnings

[JamieMagee]

FluentAssertions.Analyzers is a Roslyn analyzer for FluentAssertions. It's based on the FluentAssertions tips. Adding it to Component Detection, and fixing all of the warnings it gives will make sure we use best practices when writing test assertions.

In order to do this in a step-by-step way, FluentAssertions.Analyzers should be added to each test project individually, before being added to the test directory Directory.Build.props

• [x] Microsoft.ComponentDetection.Common.Tests
  • #694
• [x] Microsoft.ComponentDetection.Contracts.Tests
  • #634
• [x] Microsoft.ComponentDetection.Detectors.Tests
  • #879
• [x] Microsoft.ComponentDetection.Orchestrator.Tests
  • #634
• [ ] Microsoft.ComponentDetection.VerificationTests

AB#2088661

[KonH]

@JamieMagee could you please take a look at PR for Microsoft.ComponentDetection.Detectors.Tests - https://github.com/microsoft/component-detection/pull/879