Switch handwritten test serializer contexts to use source generator (#55844)

Author: layomia

src/libraries/System.Net.Http.Json/tests/FunctionalTests/JsonContext/Int32.cs

@@ -1,15 +1,11 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);net48</TargetFrameworks>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="HttpClientJsonExtensionsTests.cs" />
     <Compile Include="HttpContentJsonExtensionsTests.cs" />
     <Compile Include="JsonContentTests.cs" />
-    <Compile Include="JsonContext\Int32.cs" />
-    <Compile Include="JsonContext\JsonContext.cs" />
-    <Compile Include="JsonContext\Person.cs" />
-    <Compile Include="JsonContext\String.cs" />
     <Compile Include="TestClasses.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' == '$(NetCoreAppCurrent)'">
@@ -30,4 +26,7 @@
     <Reference Include="System.Net.Http" />
     <ProjectReference Include="..\..\src\System.Net.Http.Json.csproj" />
   </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="$(LibrariesProjectRoot)System.Text.Json\gen\System.Text.Json.SourceGeneration.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+  </ItemGroup>
 </Project>

src/libraries/System.Net.Http.Json/tests/FunctionalTests/JsonContext/JsonContext.cs

@@ -81,4 +81,9 @@ private static void AssertDefaultOptions(JsonSerializerOptions options)
 
     [JsonConverter(typeof(EnsureDefaultOptionsConverter))]
     internal class EnsureDefaultOptions { }
+
+    [JsonSerializable(typeof(Person))]
+    internal sealed partial class JsonContext : JsonSerializerContext
+    {
+    }
 }