fix warning IDE0130

Author: smdn

tests/Smdn.Net.MuninNode/Smdn.Net.MuninNode.Tests.csproj

@@ -5,6 +5,7 @@ SPDX-License-Identifier: MIT
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <Nullable>enable</Nullable>
+    <RootNamespace/> <!-- empty the root namespace so that the namespace is determined only by the directory name, for code style rule IDE0130 -->
   </PropertyGroup>
 
   <PropertyGroup>