examples/tests: Add env_variables.sh

Add an example of how avocado exposes information to
shell scripts through environment variables.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
examples/tests: Add output_check.sh

Add an example of script that will have its output
checked if executed through avocado.

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