test: should run callback with null error and environment.

Env test getEnvironment on sugar mode when env was already set: should run callback with null error and env
test: should set onEnvironmentSet handler

Env test getEnvironment on sugar mode when env not set, yet: should set environment setter
Grouping up related tests: "in sugar mode"
Refactor: common code on setup and teardown methods
force environment to be undefined
ensure env setter to be undefined before getEnvironment gets called
add test: it should run callback on EnvironmentSet event
test: it should set global sugar in standalone mode