I’ve updated the Mozilla Foundation License Policy to state that:

PD Test Code is Test Code which is Mozilla Code, which does not carry an explicit license header, and which was either committed to the Mozilla repository on or after 10th September 2014, or was committed before that date but all contributors up to that date were Mozilla employees, contractors or interns. PD Test Code is made available under the Creative Commons Public Domain Dedication. Test Code which has not been demonstrated to be PD Test Code should be considered to be under the MPL 2.

So in other words, new tests are now CC0 (public domain) by default, and some or many old tests can be relicensed as well. (We don’t intend to do explicit relicensing of them ourselves, but people have the ability to do so in their copies if they do the necessary research.) This should help us share our tests with external standards bodies.

This was bug 788511.