Debugger should provide AttachComplete event

[gregg-miskelly]

In order for the unit test runner to reliably hit breakpoints, the debugger should provide an 'AttachComplete' event that registerDebuggerAttach in src/lsptoolshost/roslynLanguageServer.ts can listen for using onDidReceiveDebugSessionCustomEvent.

Work around:

For any test you want to debug, temporarily add the following code at the start:

while (!Debugger.IsAttached)
{
    Thread.Sleep(100);
}