[Gecko Bug 1898813] Add a test for the target of coalesced and predicted events

[moz-wptsync-bot]

Differential Revision: https://phabricator.services.mozilla.com/D211614

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1898813 gecko-commit: ce1c4adf24f562062f81f1234261e1e4ef8bf9d2 gecko-reviewers: edgar