Ensure we set _solutionLoaded when the solution is loaded

I totally forgot to do this, but in practice it was hard to observe
because of https://github.com/dotnet/roslyn/issues/35372. That bug
meant that a later project creation would end up re-running the
subscription to the IVsSolution, which also checked the UIContext
and that implicitly set _solutionLoaded. Once that bug was fixed
this problem was now observable.

src/VisualStudio/Core/Def/Implementation/ProjectSystem/Legacy/SolutionEventsBatchScopeCreator.cs

@@ -193,6 +193,7 @@ int IVsSolutionEvents.OnBeforeCloseSolution(object pUnkReserved)
 
             int IVsSolutionLoadEvents.OnAfterBackgroundSolutionLoadComplete()
             {
+                _scopeCreator._solutionLoaded = true;
                 _scopeCreator.StopTrackingAllProjects();
 
                 return VSConstants.S_OK;