Add change set

Author: Westbrook

.changeset/violet-gifts-turn.md

@@ -0,0 +1,8 @@
+---
+"@web/test-runner-core": patch
+"@web/test-runner-visual-regression": minor
+"@web/test-runner": patch
+---
+
+Add `buildCache` option to the visual regression config to support always saving the "current" screenshot.
+Make the `update` option in the visual regression config _strict_, and only save "current" shots as "baseline" when it is set to `true`.