Add support for Additional Files to GenericAnalyzerTest

Author: sharwell

StyleCop.Analyzers/StyleCop.Analyzers.Test/Verifiers/GenericAnalyzerTest.cs

@@ -29,6 +29,7 @@ namespace StyleCop.Analyzers.Test.Verifiers
     using StyleCop.Analyzers.Test.Helpers;
     using TestHelper;
     using Xunit;
+    using Path = System.IO.Path;
 
     internal abstract class GenericAnalyzerTest
     {
@@ -91,6 +92,8 @@ public string TestCode
 
         public SourceFileList TestSources { get; }
 
+        public List<(string filename, string content)> AdditionalFiles { get; } = new List<(string filename, string content)>();
+
         public Dictionary<string, string> XmlReferences { get; } = new Dictionary<string, string>();
 
         public List<DiagnosticResult> ExpectedDiagnostics { get; } = new List<DiagnosticResult>();
@@ -485,6 +488,12 @@ protected virtual Solution CreateSolution(ProjectId projectId, string language)
                 .AddMetadataReference(projectId, MetadataReferences.CSharpSymbolsReference)
                 .AddMetadataReference(projectId, MetadataReferences.CodeAnalysisReference);
 
+            foreach (var (fileName, content) in this.AdditionalFiles)
+            {
+                var documentId = DocumentId.CreateNewId(projectId, fileName);
+                solution = solution.AddAdditionalDocument(documentId, Path.GetFileName(fileName), content, filePath: fileName);
+            }
+
             if (MetadataReferences.SystemRuntimeReference != null)
             {
                 solution = solution.AddMetadataReference(projectId, MetadataReferences.SystemRuntimeReference);