This adds support for assertion checking in all mochitest suites except
for mochitest-browser-chrome. The checking works much like it does in
reftest, except for the mechanism for annotating expected assertions,
SimpleTest.expectAssertions() (see its in-code documentation).
The support is initially disabled in that:
(1) It doesn't cause the tests to report failure (and thus turn the
tree orange).
(2) It prints TEST-DETCEPXENU-FAIL/PASS instead of
TEST-UNEXPECTED-FAIL/PASS (so that it doesn't show up in log
highlighting).
The assertion checking only works within the test runner (which runs
multiple tests); it does not function when running only a single test.
