Debugger: The 'debug test' is not disabled in debugging

## Environment data
`dotnet --info` output:
Product Information:
`Version: 1.0.1`
`Commit SHA-1 hash: 005db40cd1`
Runtime Environment:
`OS Name: Ubuntu`
`OS Version: 16.10`
`OS Platform: Linux`
`RID: Ubuntu.16.10-x64`
`Base Path: /usr/share/dotnet/sdk/1.0.1`

VS Code version: v1.11.2
C# Extension version: v.1.9.0

## Steps to reproduce
1. Open attached folder with VSCode;
[test_xunit.zip](https://github.com/OmniSharp/omnisharp-vscode/files/950510/test_xunit.zip)
2. Set a breakpoint at the line of 11 in UnitTest1.cs;
3. Debug test and wait for break mode;

## Expected behavior
The `debug test` should be disabled.

## Actual behavior
The `debug test` is not disabled. And when I click `debug test` again, pop an error message as following:

## Investigation
1. In VS, `Run Tests`, `Debug Tests` and `Restart` was be disabled if the project is being debugged.
2. In VSCode, click `Run Tests` after step #3, .NET Test Log as following:
3. In VSCode, click `Restart` after step #3, the project run to end. Could you please help me to confirm if it is expected? OmniSharp Log as following:
4. If `debug test` can not be disabled, can you improve the prompt? E.g: Failed to start debugger: Is being used by another process.