Simplify xUnit Migration Code Fixer (#2452)

* Don't include global usings if xunit is still installed as it may break the migration code fixer

* Combine analyzers

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* Test fixes

* Fix

* Fix

* Update xunit.md

* WIP

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* Passing

* Working!

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* Test fixes

Author: thomhurst

TUnit.Analyzers.CodeFixers/Extensions/DocumentExtensions.cs

@@ -16,7 +16,7 @@ public static async Task<CompilationUnitSyntax> AddUsingDirectiveIfNotExistsAsyn
         }
 
         var usingDirective = SyntaxFactory.UsingDirective(SyntaxFactory.ParseName(namespaceName).WithLeadingTrivia(SyntaxFactory.Space))
-            .WithTrailingTrivia(RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? SyntaxFactory.ElasticCarriageReturnLineFeed : SyntaxFactory.ElasticLineFeed);
+            .WithTrailingTrivia(SyntaxFactory.ElasticCarriageReturnLineFeed);
 
         return root.AddUsings(usingDirective);
     }