ci: set continue-on-error: true on test-ui (#17646)

Since the matrix exercises different test cases, it's better to allow
all partitions to completely run, even if one of them fails, so it's
easier to catch multiple test failures.

.github/workflows/test-ui.yml

@@ -61,6 +61,7 @@ jobs:
       - pre-test
     runs-on: ubuntu-latest
     timeout-minutes: 30
+    continue-on-error: true
     defaults:
       run:
         working-directory: ui