add NetCoreAppCurrent to TargetFrameworks on TestApp

Author: maryamariyan

src/libraries/Microsoft.Extensions.Hosting/tests/TestApp/Microsoft.Extensions.Hosting.TestApp.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>$(NetFrameworkCurrent)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkCurrent)</TargetFrameworks>
     <EnableDefaultItems>true</EnableDefaultItems>
     <OutputType>Exe</OutputType>
   </PropertyGroup>