Add filesystem:: and filesystem::exchange, fix - assert_file creates an empty file...

Add filesystem:: and filesystem::exchange, fix - assert_file creates an empty file if it not yet exists. Parent dirs
  are created as needed.
- exchange exchanges two files or directories located on the same
  filesystem. Mainly used when creating new config files in a temp
  file and later exchanging it with the original one.
- fixed assert_dir returning OK, if the path alsready exists but isn't a
  directory.
- updated testcases