Currently the xpcshell tests use a private API and luck to implement a custom debugging feature that allows developers to attach an external debugger to a test and pause at the start of the test file before it has executed. We'd like to remove some of the APIs that it relies on in https://bugzilla.mozilla.org/show_bug.cgi?id=1513655 so we need to update the test setup first.

Pushed by lsmyth@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/72065a75fc01 Reimplement xpcshell test debugging using a public thread-actor API. r=jlast