CodeLens not registering derived classes of abstract test classes as runnable (XUNIT)

## Issue Description ##

Working on writing tests, and have a situation where I have reusable test code that I want to run with different fixtures (managing different user configurations). To manage this, I'm making an abstract *TestsBase class with all tests listed in it, and 2 wrapper classes with different fixture. Here is an example layout similar to what I'm trying to do (4 tests in base class):

Screenshot from VSCode


When I run tests via dotnet test or a test runner (VS Test Explorer, and [this VS Code Extension](https://marketplace.visualstudio.com/items?itemName=formulahendry.dotnet-test-explorer) (recommended by Joey R.), it discovers all tests (8 total), and the test explorer shows a correctly organized view:

Test Explorer Screenshot


However, the Code Lens Run/Debug All Tests are not connecting to the wrappers with Fixtures (see first picture). The Run/Debug on the base class doesn't return any tests, since it is an abstract class.

## Steps to Reproduce ##

Make an abstract class with 1/many public test in it (4 in example). Make 2 public wrapper classes (put traits/IClassFixtures on them to recreate). Running dotnet test will run 2x the amount of tests in your base class (8 tests ran in my example).

Code snippet

```
using System;
using Xunit;

namespace Component.Tests
{
[Trait("Category","SQLite")]
public class SQLiteWrapper : UnitTest1, IClassFixture
{
public SQLiteWrapper(SqliteFixture sut) : base(sut){ }
}

[Trait("Category","SqlServer")]
public class SqlServerWrapper : UnitTest1, IClassFixture
{
public SqlServerWrapper(SqlServerFixture sut) : base(sut){ }
}

public abstract class UnitTest1
{
public UnitTest1(IProvider cst)
{
sut = cst;
}
IProvider sut;
[Fact]
public void Test1()
{
Assert.Equal(1,1);
}

[Theory]
[InlineData(1)]
public void Test2(int expected)
{
Assert.Equal(expected,sut.MyInt);
}

[Theory]
[InlineData(2)]
public void Test3(int expected)
{
Assert.Equal(expected,sut.OurInt);
}

[Fact]
public void Test4()
{
int sum = sut.MyInt + sut.OurInt;

Assert.Equal(3,sum);
}
}
}
```

## Expected Behavior ##

Code Lens Run/Debug All Tests Option should connect to derived classes that inherit from a class with Tests in it.

## Actual Behavior ##

Code Lens only connects to the base class, even if it's abstract. It does not connect to the derived classes.

## Logs ##

Didn't grab since I don't think it's necessary, but I can come back and upload if needed.

## Environment information ##

**VSCode version**: 1.48.0
**C# Extension**: 1.23.1

Dotnet Information
.NET Core SDK (reflecting any global.json):
Version: 3.1.400
Commit: 035fb2aa2f

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19041
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.400\

Host (useful for support):
Version: 3.1.6
Commit: 3acd9b0cd1

.NET Core SDKs installed:
3.1.400 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|cpptools|ms-vscode|0.29.0|
|csharp|ms-dotnettools|1.23.1|
|csharpextensions|jchannon|1.3.1|
|debugger-for-edge|msjsdiag|1.0.15|
|docomment|k--kato|0.1.18|
|dotnet-test-explorer|formulahendry|0.7.4|
|EditorConfig|EditorConfig|0.15.1|
|github-sharp-theme|joaomoreno|1.4.0|
|markdown-all-in-one|yzhang|3.2.0|
|remote-containers|ms-vscode-remote|0.134.0|
|remote-ssh|ms-vscode-remote|0.51.0|
|remote-ssh-edit|ms-vscode-remote|0.51.0|
|remote-wsl|ms-vscode-remote|0.44.4|
|rest-client|humao|0.24.1|
|vscode-arduino|vsciot-vscode|0.3.1|
|vscode-docker|ms-azuretools|1.5.0|
|vscode-drawio|hediet|0.7.2|
|vscode-remote-extensionpack|ms-vscode-remote|0.20.0|
|vscode-xml|redhat|0.13.0|
|vscodeintellicode|VisualStudioExptTeam|1.2.10|
|vsliveshare|ms-vsliveshare|1.0.2666|
|vsliveshare-audio|ms-vsliveshare|0.1.85|
|vsliveshare-pack|ms-vsliveshare|0.4.0|
|vsonline|ms-vsonline|1.0.2669|;

## Unrelated note

This is honestly the smoothest "Report an issue" workflow I've ever experienced. Kudos.