OmniSharp can't Run or Debug inherited NUnit Tests

## Issue Description ##
If you have an abstract class which defines some tests and you inherit from that class to a concrete class. Those tests will not run using the 'Run Test'|'Debug Test' options above the method. (Makes sense this is an abstract class ) but you also can't run it from the concrete class either (there is no option as the method is not defined in the concrete class).

These tests do run fine if you use dotnet run.

I did get the test to show up in the '.NET Core Test Explorer' Extension if I add a [TestFixture] attribute to the concrete class. However if you try and debug it just shows a spinner forever and never runs the test.

As far as I can tell there is no way to debug these tests using OmniSharp

## Environment information ##

**VSCode version**: 1.57.1
**C# Extension**: 1.23.12

Mono Information
OmniSharp using built-in mono
Dotnet Information
.NET SDK (reflecting any global.json):
Version: 5.0.301
Commit: cc8c0abb32

Runtime Environment:
OS Name: ubuntu
OS Version: 18.04
OS Platform: Linux
RID: ubuntu.18.04-x64
Base Path: /usr/share/dotnet/sdk/5.0.301/

Host (useful for support):
Version: 5.0.7
Commit: 556582d964

.NET SDKs installed:
5.0.301 [/usr/share/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 5.0.7 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 5.0.7 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

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

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|csharp|ms-dotnettools|1.23.12|
|docomment|k--kato|0.1.20|
|dotnet-test-explorer|formulahendry|0.7.5|
|remote-ssh|ms-vscode-remote|0.65.7|
|remote-ssh-edit|ms-vscode-remote|0.65.7|
|ShowTestResults|EffectiveLabs|0.4.0|;