This test just recently started failing on macOS Mojave wk2 on release. There are some failures on other platforms way further back in the history, but I am not are they are related. This was first seen on commit 255615.
@@ -1,4 +1,4 @@
-PASS Ensuring frame document gets collected after being stopped in the middle of IDB operations
+FAIL Ensuring frame document gets collected after being stopped in the middle of IDB operations promise_test: Unhandled rejection with value: "Test failed"
set expectations here: https://trac.webkit.org/changeset/256333/webkit