Ensure IsTestProject=false in Microsoft.AspNetCore.Identity.Specification.Tests.csproj

This isn't really a test project. It's a class library which contains testing compontents. Because 'xunit' is referenced, IsTestProject=true, which
messes up some of the build logic we have for handling code signing and test runners

src/Specification.Tests/Microsoft.AspNetCore.Identity.Specification.Tests.csproj

@@ -6,6 +6,7 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;identity;membership</PackageTags>
     <IsPackable>true</IsPackable>
+    <IsTestProject>false</IsTestProject>
   </PropertyGroup>
 
   <ItemGroup>