[add] include Simplify.Mail.IntegrationTests project in solution

Author: i4004

src/Simplify.NetCore.slnx

@@ -57,6 +57,7 @@
     <Project Path="Simplify.IO/Simplify.IO.csproj" />
   </Folder>
   <Folder Name="/Mail/">
+    <Project Path="Simplify.Mail.IntegrationTests/Simplify.Mail.IntegrationTests.csproj" />
     <Project Path="Simplify.Mail.TestConsoleApp/Simplify.Mail.TestConsoleApp.csproj" />
     <Project Path="Simplify.Mail/Simplify.Mail.csproj" />
   </Folder>