tests/integration/shell.py: Added tests for host-files

Newly added tests:

  o Test that bind mounting a file works

  o Test that bind mounting a file into a non-existing directory works

  o Test that bind mounting is disabled in an isolated shell

  o Test that the shell still works, but a warning is printed and
    the mount is refused in case the specified file on the host
    was found to be a directory

  o Test that the shell still works, but a warning is printed
    in the case that the file specified on the host does not exist