Convert Pathname objects to strings on suite load

This could be another edge case, but for some reason, when loading my test suite, I was getting a "TypeError: can't convert Pathname to String". In my efforts to debug I found that, in lib/suite.rb#asset_from_file, when it was cycling through 'path', at least one Pathname object was coming through. It seemed like an easy solution to just convert to string before attempting the gsub on the next line.