Add IsTestProject per dotnet SDK requirement

Author: AArnott

src/NerdBank.GitVersioning.Tests/NerdBank.GitVersioning.Tests.csproj

@@ -6,6 +6,7 @@
     <DebugType>full</DebugType>
     <SignAssembly>false</SignAssembly>
     <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
   </PropertyGroup>
   <ItemGroup>
     <EmbeddedResource Include="..\Nerdbank.GitVersioning.Tasks\build\Nerdbank.GitVersioning.targets">
@@ -39,4 +40,4 @@
   <ItemGroup>
     <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" />
   </ItemGroup>
-</Project>
+</Project>