Add more unit tests for pasing files and test cases where the input is
invalid. In particular, add the following tests:
- Invalid Containerfile, which is empty and can not be parsed
- Invalid file, which contains a single line

Signed-off-by: Charalampos Mainas <cmainas@nubificus.co.uk>

Add one more unit test to verify that the creation of a rootfs with an
invalid format in the include files will fail.

Signed-off-by: Charalampos Mainas <cmainas@nubificus.co.uk>

Add further unit tests for checking the unsupported features that a
rootfs field implies in a bunnyfile.
In particular add the following tests:
- Non-empty path with raw rootfs type, which means that we specify both
  a file to use as rootfs but also set the type as raw, which will
  require to unpack the file and transform it to raw. This is not
  supported yet.
- The from field equals "local" and the rootfs type is raw, which means
  that we have to use something local as raw, which is similar to the
  previous case (use a file with raw rootfs type). Again not supported
  yet.
- The from field is not scratch and we the include field is non-empty,
  which means that we want to append files in an existing rootds. This
  is not supported yet.

Signed-off-by: Charalampos Mainas <cmainas@nubificus.co.uk>