avocado.test: Drop 'actual' suffix on test stdout/stderr files

As pointed out during review, it is more straightforward
to just call the stream files 'stdout' and 'stderr'.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.test: Refactor logger setup code

As pointed out during code review by Rudá.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Update WritingTests with feedback from first review

Updates the docs with all behavior changes and comments
on text structure/phrasing.

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