Disable the virtual file system when running tests

The VFS is implemented in FCS as a static singleton, hence this ugliness. We want to ensure that the VFS initialisation only occurs once, so it is now lazy. 
The tests need to call `MDLanguageService.DisableVirtualFileSystem` before anything reaches `MDLanguageService.Instance` and a language service is created.
I don't remember it being lazy previously, but you're probably right. Now it does serve a purpose.  The natural point to create the VFS seems to me to be when creating the language service. With it being lazy, we can make sure to disable creation of VFS before accessing the Language service for the first time.