Add missing csproj

Author: Evangelink

docs/core/testing/snippets/testcontext/csharp/project.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="MSTest" Version="3.6.3" />
+  </ItemGroup>
+
+</Project>