Previously, set_muted_topics was calling update_unread_counts once for each
topic in the input; this results in poor performance when there is a large
number of muted topics.
Fixes: #3605
* In most cases, eslint --fix with the right comma-dangle settings was
able to update the code correctly.
* The exceptions were cases where the parser incorrectly treated the
arguments to functions like `assert_equal` as arguments; we fixed
these manually. Since this is test code, we can be reasonably
confident that just fixing the failures suffices to correct any bugs
introduced by making changes automatically.