[anchor] Add tests for anchor-scope and :host/::slotted.
Fixed: 368982570
Change-Id: I4bec062e0ffa0fb2fc3385c3551c29fbeeb5b746
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5886350
Commit-Queue: Anders Hartvoll Ruud andruud@chromium.org
Reviewed-by: Rune Lillesveen futhark@chromium.org
Cr-Commit-Position: refs/heads/main@{#1359776}