tracing (via set_trace_func) is started too early, and not cleared between tests

[alainravet]

• In auto-mode, the tracer should be started when the gem loads, and never be cleared.
• In manual mode, the tracer should be started just after the 1st call to a private_please so the required data is available when the 1st instrumented method is triggered (otherwise it wouldn't know anything about the call initiator).