Fix regressed excel testcase

When ``Marshal.GetActiveObject("Excel.Application")`` is called for the first time, a mysterious ``System.NotSupportedException`` "The URI prefix is not recognized" will be thrown out. Following is stack trace:

   at System.Net.WebRequest.Create(Uri requestUri, Boolean useUriBase)
   at MS.Internal.WpfWebRequestHelper.CreateRequest(Uri uri)
   at System.IO.Packaging.PackWebRequest.GetRequest(Boolean allowPseudoRequest)
   at System.IO.Packaging.PackWebRequest.GetResponse()
   at MS.Internal.WpfWebRequestHelper.GetResponse(WebRequest request)
   at MS.Internal.WpfWebRequestHelper.CreateRequestAndGetResponse(Uri uri)
   at MS.Internal.FontCache.FontSource.GetUnmanagedStream()
   at MS.Internal.Text.TextInterface.FontFileStream..ctor(IFontSource fontSource)
   at MS.Internal.Text.TextInterface.FontFileLoader.CreateStreamFromKey(Void* fontFileReferenceKey, UInt32 fontFileReferenceKeySize, IntPtr* fontFileStream)

 Unfortunately I have no idea why that happens. :-( As a new excel instance will always be created immediately for the first run, here just swallow this exception.

@aparajit-pratap for you.