Cannot use vscode variables for dotnet-test-explorer.pathForResultFile

Hello, I see it's not possible to use vscode variables into dotnet-test-explorer.pathForResultFile configuration.

In my case, I use `"dotnet-test-explorer.pathForResultFile": "${workspaceFolder}",`
In vscode console I have this error:
`Activating extension 'formulahendry.dotnet-test-explorer' failed: ENOENT: no such file or directory, mkdtemp 'c:\Developpement\Base\${workspaceFolder}\test-explorer-XXXXXX'`

If I comment `"dotnet-test-explorer.pathForResultFile": "${workspaceFolder}",` and I refresh test discovering, I have this error :
```
Error: command 'dotnet-test-explorer.refreshTestExplorer' not found
at Proxy._tryExecuteCommand (commandService.ts:60)
at r.then.i (commandService.ts:53)
at process._tickCallback (internal/process/next_tick.js:68)
```

Thanks.