Improve coverage for SA1304 by adding test for a field in a native methods class

Author: bjornhellander

StyleCop.Analyzers/StyleCop.Analyzers.Test/NamingRules/SA1304UnitTests.cs

@@ -61,6 +61,17 @@ public async Task TestProtectedReadonlyFieldStartingWithUpperCaseAsync()
             await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
         }
 
+        [Fact]
+        public async Task TestFieldInNativeClassMethodAsync()
+        {
+            var testCode = @"public class FooNativeMethods
+{
+    internal readonly string bar = ""baz"";
+}";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
+
         [Fact]
         public async Task TestInternalReadonlyFieldStartingWithLowerCaseAsync()
         {