Feature: return source generator with helper (#4)

* Feature: return source generator with helper

* Add the missing documentation

Author: glennawatson

src/ReactiveMarbles.SourceGenerator.TestNuGetHelper/Compilation/SourceGeneratorUtility.cs

@@ -72,17 +72,22 @@ public SourceGeneratorUtility(Action<string> writeOutput)
         /// <param name="compilationDiagnostics">The diagnostics which are produced from the compiler.</param>
         /// <param name="generatorDiagnostics">The diagnostics which are produced from the generator.</param>
         /// <param name="sources">The source code files.</param>
-        public void RunGenerator<T>(EventBuilderCompiler compiler, out ImmutableArray<Diagnostic> compilationDiagnostics, out ImmutableArray<Diagnostic> generatorDiagnostics, params string[] sources)
+        /// <returns>The source generator instance.</returns>
+        public T RunGenerator<T>(EventBuilderCompiler compiler, out ImmutableArray<Diagnostic> compilationDiagnostics, out ImmutableArray<Diagnostic> generatorDiagnostics, params string[] sources)
             where T : ISourceGenerator, new()
         {
             var compilation = CreateCompilation(compiler, sources);
 
+            var generator = new T();
+
             var newCompilation = RunGenerators(compilation, out generatorDiagnostics, new T());
 
             compilationDiagnostics = newCompilation.GetDiagnostics();
 
             ShouldHaveNoCompilerDiagnosticsWarningOrAbove(_writeOutput, newCompilation, compilationDiagnostics);
             ShouldHaveNoCompilerDiagnosticsWarningOrAbove(_writeOutput, compilation, generatorDiagnostics);
+
+            return generator;
         }
 
         private static void ShouldHaveNoCompilerDiagnosticsWarningOrAbove(Action<string> output, Microsoft.CodeAnalysis.Compilation compilation, IEnumerable<Diagnostic> diagnostics)