Reference Target tentative tests

Tests in this directory are for the proposed Reference Target feature for shadow dom. This is not yet standardized and browsers should not be expected to pass these tests.

See the explainer at https://github.com/WICG/aom/blob/gh-pages/reference-target-explainer.md for more information about the API.

These are blink-specific tests because they rely on accessibilityController. There are also web platform tests for Reference Target located here: third_party/blink/web_tests/external/wpt/shadow-dom/reference-target/.