Debugging Unit Test loses codelens Run Test / Debug Test - DebugSessionManager does not stop

Related to #1743

## Environment data
`dotnet --info` output:

.NET Core SDK (reflecting any global.json):
Version: 2.1.301
Commit: 59524873d6

Runtime Environment:
OS Name: Windows
OS Version: 10.0.17134
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.1.301\

Host (useful for support):
Version: 2.1.1
Commit: 6985b9f684

.NET Core SDKs installed:
1.0.4 [C:\Program Files\dotnet\sdk]
2.0.2 [C:\Program Files\dotnet\sdk]
2.0.3 [C:\Program Files\dotnet\sdk]
2.1.2 [C:\Program Files\dotnet\sdk]
2.1.3 [C:\Program Files\dotnet\sdk]
2.1.4 [C:\Program Files\dotnet\sdk]
2.1.100 [C:\Program Files\dotnet\sdk]
2.1.101 [C:\Program Files\dotnet\sdk]
2.1.102 [C:\Program Files\dotnet\sdk]
2.1.103 [C:\Program Files\dotnet\sdk]
2.1.104 [C:\Program Files\dotnet\sdk]
2.1.105 [C:\Program Files\dotnet\sdk]
2.1.200 [C:\Program Files\dotnet\sdk]
2.1.201 [C:\Program Files\dotnet\sdk]
2.1.300 [C:\Program Files\dotnet\sdk]
2.1.301 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download
PS D:\CBRock\XCellorator>

VS Code version: 1.24.1
C# Extension version:
ms-vscode.csharp-1.15.2\.omnisharp\1.30.1\OmniSharp.exe

## Steps to reproduce
Create a TestServer that has a IHostedService with dependencies and do not perform a graceful close on the TestServer using the following code: ` this.testServerIam.Host.StopAsync().GetAwaiter().GetResult();`. The `OmniSharp.DotNetTest.DebugSessionManager` will never report "Debug session ended", it will be stuck with "Debug session started.", even though the .NET Test Log says Debugging complete. The extra host process not being cleaned up / shutdown must be confusing omnisharp. In some cases the codelens may appear on a fresh restart but clicking the debug again will just hang with the debug starting in the output window (it will never start).

## Expected behavior
Omnisharp debug session stops, codelens run/debug appears and a subsequent debug can be performed.