Include libyaml-test and libyaml-parser-emitter

Before the `make test-suite` command would clone yaml/libyaml-test which
would in tuen clone yaml/libyaml-parser-emitter. Now they are part of
the repository since they are not really useful outside the repository.

They still clone the data branch of yaml/yaml-test-suite. That is where
all the actual test data resides. That belongs outside the repo because
it is intended for all yaml frameworks.