hermes-utils.js: Add test coverage

Summary:
Improving test coverage in anticipation of some changes being made to `hermes-utils.js`.

Moved test scripts to `hermes/__tests__` and grouped related tests.

We have been delegating some of the work to local binaries via `execSync`, which can be hard to mock or test.  We now use a proxy `delegateSync` method that uses `spawnSync` internally to break down the invocation into `command`, `arguments`, `options`. Instead of simply mocking based on the command being executed, we can now conditionally mock based on the arguments being passed.

Added a `createTarballFromDirectory` method. This can be used later when creating different tarballs.

Added `populateMockFilesystemWithHermesBuildArtifacts()` to mock the filesystem state after Hermes has been built.

Changelog: [internal]

Reviewed By: cipolleschi

Differential Revision: D40871802

fbshipit-source-id: 4348d3c38926ec7eb13d794040a9040010879f58

Author: hramos