This patch moves some "devtools related" test helpers into a shared support file, and updates all the existing devtools API mochitests to use the new shared test helpers. This change has also the following nice side-effects: - remove any usage of the custom devtools module loader from the webextensions mochitests. - use a custom "test blank toolbox panel" in all the devtools API tests that doesn't need a specific panel to be selected (which should help to make the test able to run a bit faster because none of the more expensive devtools panels is going to be loaded during the tests if not actually needed).

This patch applies the following changes: - make sure that the WebExtensions internals do not create any devtools_page related to a private tabs' toolbox if the extension is not allowed to access the private browsing windows - define a new mochitest to verify the expended behaviors on the private windows when the extension is allowed and not allowed Depends on D15948

Depends on D15949

Pushed by luca.greco@alcacoop.it: https://hg.mozilla.org/integration/autoland/rev/f5b40e3836f1 Move extension "devtools related" test helpers into a shared head_devtools.js support file. r=mixedpuppy https://hg.mozilla.org/integration/autoland/rev/c63e693b1ce2 Disable extension devtools_page for private tabs when not allowed by users. r=mixedpuppy https://hg.mozilla.org/integration/autoland/rev/5781d37b8a3b Reuse shared loadTestSubscript test helper in other extensions mochitest-browser test files. r=mixedpuppy