* init/job_class.c: job_class_environment_clear(): Utility function
  exposed primarily to allow tests to invalidate job_environ.
* init/tests/test_state.c:
  - Added appropriate calls to job_class_environment_clear().
  - New JSON data tests added:
    - upstart-no-job-environ.json
    - upstart-with-job-environ.json
    - session-init-no-job-environ.json
    - session-init-with-job-environ.json
    - session-init-with-modified-job-environ.json