Bug 979640 - Make build system error on missing test files; r=ted

Previously, the build system may silently missing test files defined in
manifests. This patch makes missing test files a fatal error, detected
when reading test manifests.

The test_bug872273.html XBL test appeared to be orphaned in
content/xbl/test. It has been reunited with its family.

dom/tests/mochitest/notification referenced a single test file which was
recently deleted. That manifest has been removed.

Missing test files related to the Python unit tests for the build system
have been added. (They are a bunch of empty files.)

--HG--
extra : rebase_source : 8c64986169064401951585c07deadada8c905550