avocado: Introduce --output-check-record option to test runner

It is a common use case in test automation that we want
to check the output of the test or applications started
by it and compare it with a reference output.

Introduce the option --output-check-record, that will
run the test you specify and generate the files
'stdout.expected' and 'stderr.expected' in the test
data dir (same directory as the source file, with the
name 'filename.data').

In case that option is not provided, if there is an
expected stdout or stderr file, those files will be
checked against the output the test just produced. If
no files are found, the check will be skipped. If
--output-check-record, no check will be made on that
particular execution, since we are recording what
is going to be expected in future test runs.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
examples/tests/synctests: Add stdout and stderr reference files

These files were captured using our new runner option

$ avocado run --output-check-record all examples/tests/synctest.py

And should now be checked every time that particular test
will run.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Add Output Check Mode documentation

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