Introduce clar_test_init and clar_test_shutdown

Introduce clar_test_init and clar_test_shutdown so that callers
can perform some actions after initializing the clar system
(sandbox, etc) but before test execution.
Introduce clar_sandbox_path()

Callers can inspect the sandbox path for population before/after
test execution.
Merge pull request #49 from ethomson/init_shutdown

Init shutdown