Before this patch, experiment add-ons may have existed in the Addons
Manager without the Experiments service knowing about them. This detects
these unknown add-ons and uninstalls them. See the in-line comment on
the rationale behind this decision.
The added unit test fails without the Experiments.jsm change.
* no need to pass an addon/experiment ID to .disableExperiment()
* fix multiple-logging when multiple Experiments() objects are created as in tests
* ensures that all dirty writes actually get written
