Since we don't enable the preference as part of
`--enable-experimental-web-platform-features` yet, we don't run any
automated tests for the AbortController feature. This change means that
we at least explicitly test the interface.
Testing: Just enabling new tests.
Part of: https://github.com/servo/servo/issues/34866
---------
Signed-off-by: Josh Matthews <josh@joshmatthews.net>
