merge_pr_48136

WPT tests for position-try-fallbacks should use canonical ordering.

Spec 1 defines "Canonical order: per grammar," so the serialization should
re-order <try-tactics> entries in order of their appearances 2.

Depends on D221988

Differential Revision: https://phabricator.services.mozilla.com/D221989

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1918419
gecko-commit: 48ce410c6840ff88a36d7482eb078df724936af9
gecko-reviewers: firefox-style-system-reviewers, emilio