avocado.test: Define a 'test-results' toplevel directory

Inside the job result instance dir, introduce a 'test-results'
directory that holds the results of all the other test instances.
This organizes things better and is a standard dir name,
since now we mimic a the origin test directory structure.

Update the docs accordingly.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>