xUnit Test Project (.NET Core) use the old project type GUID in the Solution file

**Visual Studio Version**: Microsoft Visual Studio Community 2019, Version 16.6.3

**Summary**: When creating a new project of type `xUnit Test Project (.NET Core) C#`, the project is added to the solution with the GUID `FAE04EC0-301F-11D3-BF4B-00C04F79EFBC` (legacy C#) instead of the new GUID [`9A19103F-16F7-4668-BE54-9A1E7A4F7556`](https://github.com/dotnet/project-system/blob/master/docs/opening-with-new-project-system.md#project-type-guids) (SDK-style C#).

**Steps to Reproduce**:

1. Open Visual Studio 2019, Version 16.6.3

2. Create a new project of type `xUnit Test Project (.NET Core) C#` with the name `Project1`

3. Open the solution file (`.sln`) on a text editor and notice that project is identified as being a legacy C# project

**Expected Behavior**:

```
MinimumVisualStudioVersion = 10.0.40219.1
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Project1", "Project1\Project1.csproj", "{9431EF13-2586-440E-B323-1145C81F6672}"
EndProject
```

**Actual Behavior**:

```
MinimumVisualStudioVersion = 10.0.40219.1
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Project1", "Project1\Project1.csproj", "{9431EF13-2586-440E-B323-1145C81F6672}"
EndProject
```

**User Impact**:

- Tooling incorrectly identify xUnit projects as being legacy C# projects instead of SDK-style projects
- Visual Studio 2019 has different behavior depending on the project type GUID ([example](https://developercommunity.visualstudio.com/content/problem/474512/when-adding-an-existing-project-to-solution-vs2019.html))
- Visual Studio changes the GUID to `9A19103F-16F7-4668-BE54-9A1E7A4F7556` at a later time (it's not clear exactly when) which creates changes in source control that are not related to the work being done at the time