Use proper C# decimal separator '.' in TestIncrementCompilationAndOutput

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenIncrementTests.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
-using Microsoft.CodeAnalysis.CSharp.Symbols;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
+using System.Globalization;
 using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
-using Microsoft.CodeAnalysis.CSharp.UnitTests.Emit;
-using Microsoft.CodeAnalysis.Text;
 using Roslyn.Test.Utilities;
 using Xunit;
 
@@ -1001,8 +998,9 @@ static int[] Array()
             Type type = typeof(T);
             Assert.True(type.IsPrimitive || type == typeof(decimal), string.Format("Type {0} is neither primitive nor decimal", type));
 
-            string source = string.Format(NUMERIC_INCREMENT_TEMPLATE, type.FullName, value, valuePlusOne);
-            string expectedOutput = string.Format(NUMERIC_OUTPUT_TEMPLATE, value, valuePlusOne);
+            // Explicitly provide InvariantCulture to use the proper C# decimal separator '.' in the source regardless of the current culture
+            string source = string.Format(CultureInfo.InvariantCulture, NUMERIC_INCREMENT_TEMPLATE, type.FullName, value, valuePlusOne);
+            string expectedOutput = string.Format(CultureInfo.InvariantCulture, NUMERIC_OUTPUT_TEMPLATE, value, valuePlusOne);
 
             CompileAndVerify(source, expectedOutput: expectedOutput);
         }