Summary:
This diff adds a minimal workflow for running integrations tests for Mononoke. Currently only one test is run and it fails.

This also splits the regular Mononoke CI into separate files for Linux and Mac to match the current style in Eden repo.
There are the "scopeguard::defer" fixes here that somehow escaped the CI tests.
Some tweaks have been made to "integration_runner_real.py" to make it runnable outside FB context.
Lastly the change from using "[[ -v ... ]" to "[[ -n "${...:-}" ]]; in "library.sh" was made because the former is not supported by the default Bash version preinstalled on modern MacOS.

Pull Request resolved: https://github.com/facebookexperimental/eden/pull/26

Reviewed By: krallin

Differential Revision: D22541344

Pulled By: lukaspiatkowski

fbshipit-source-id: 5023d147823166a8754be852c29b1e7b0e6d9f5f