Update Gemfile + test env variable
 make Scheduler synchronuous for tests
 Add Raclette::HTTP tests
 Add stub of Raclette::BaseOptions
 Test: create VoidObject and import a CountdownLatch
 Add Raclette::AtomicID tests
 Add Raclette::CentralStorage tests
 Add Raclette::MultiIO tests
 Add Raclette::Pipeout tests
 Add *basic* Raclette::Scheduler tests. TODO: complete