Meziantou.FluentAssertionsAnalyzers is a Roslyn analyzer to help migrate from Xunit or NUnit assertions to Fluent Assertions.
Install the NuGet package: Meziantou.FluentAssertionsAnalyzers
You can exclude assertion methods using the .editorconfig file:
[*.cs]
mfa_excluded_methods=M:NUnit.Framework.Assert.Fail;M:NUnit.Framework.Assert.Fail(System.String)