Move bluetooth-helpers.js into mozilla test directory

I only noticed via https://github.com/w3c/web-platform-tests/pull/4178/ that bluetooth-helpers.js was added to tests/wpt/web-platform-tests/bluetooth/ rather than tests/wpt/mozilla/tests/mozilla/bluetooth/.

cc @zakorgy @dati91 