refactor `dump_failures` script to have an excluded manifest list instead of harcoded in the middle of the logic

in dump_failures.py, there is some code like this:

            if manifest in ("dom/canvas/test/mochitest.ini",):
                continue

the reason for this is the format of test names and output is not supported yet (due to being a sub test suite) in dump_failures, so until we have logic to handle these sub test suites it is simpler to skip these as seen in the current code.

To solve this issue here are some recommendations:

1. build a list of manifests that are defined at the top of the file in a constant (SKIPPED_MANIFESTS = ...)
2. replace the current code to use the new list of defined tests
3. add a comment in code to help out with this, update any documentation as needed