positioner: various fixes for tests and positioner

• Make sure tst_default_positions is reset properly
• In checkDefaultPositions compare to positioner rowcount, since foldermodel may not hold the items in same order
  • foldermodel order seemed to be highly dependant on system, for example if this is ran locally it always succeeds
• skip empty urls in checkDefaultPositions
• remove proxySource.empty test from setPerStripe
  • convertFolderModelData will clear proxySource anyway
  • this ensures convertFolderModelData is called when there is no errors, and it will make sure things are synced during perStripe changes
• in convertFolderModelData ensure that positions are m_positions.count() <= 2
  • the first two values are amount of stripes, and icons per stripe
• use m_positions.mid(2); instead of std::span
  • std::span caused ILLEGAL error code on freebsd CI