More informative log message when a test executable is filtered out

Hi,
I think that "Invalid source" log message found in function `Discoverer.GetTests` could be more informative.

I had to look for it in the source code to figure out that it means something like: 

"The executable 'XXX.exe' has been ignored because it does not match the filter specified in 'FilenameFilter' parameter of the '.runsettings' file"

Just a suggestion 😄 