Currently code used by many mochitest-browser tests is scattered
throughout the tree in various head.js files. Many similar or identical
helper methods are repeated throughout these files.
This commit introduces a BrowserTestUtils.jsm module and includes it in
the mochitest scope; the idea being these frequently re-implemented
methods can live in a central place.
A TestUtils.jsm module has also been introduced to contain code useful to
all types of tests.
