[sdk_v2] disables generation of apphost.exe for test suite (#528)

This works since dotnet test directly runs the generated testing .dll so
the native .exe is not needed.

```
C:\Program Files\dotnet\sdk\10.0.201\Microsoft.Common.CurrentVersion.targets(5395,5): error MSB3030: Could not copy the file 
"D:\a\Foundry-Local\Foundry-Local\sdk_v2\cs\test\FoundryLocal.Tests\obj\Debug\net9.0\win-x64\apphost.exe" because it was not 
found. [D:\a\Foundry-Local\Foundry-Local\sdk_v2\cs\test\FoundryLocal.Tests\Microsoft.AI.Foundry.Local.Tests.csproj]
```

Co-authored-by: Prathik Rao <prathikrao@microsoft.com>

Author: prathikr

sdk_v2/cs/test/FoundryLocal.Tests/Microsoft.AI.Foundry.Local.Tests.csproj

@@ -7,6 +7,7 @@
     <IsPackable>false</IsPackable>
     <IsWindows Condition="$([MSBuild]::IsOSPlatform('Windows'))">true</IsWindows>
     <UseWinML>false</UseWinML>
+    <UseAppHost>false</UseAppHost>
   </PropertyGroup>
 
   <ItemGroup>