`head.js` is auto-loaded by browser-chrome for tests in the same directory, which collides with its role as a helper file that subdirectory suites pull in explicitly via `loadSubScript`. We move the shared helpers to `head-common.js` and update all consumers. `tests/head.js` remains as a thin shim that loads `head-common.js`, preserving auto-loading for tests directly under `tests/`. Differential Revision: https://phabricator.services.mozilla.com/D309388