Change failing test list predicate to UNEXPECTED_MASK

This fixes an issue where flaky tests were not showing up in the
reported list.

Change-Id: I2de5510c530c4ba5258681e65de514348f9a47d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/7716124
Auto-Submit: Russ Hamilton <behamilton@google.com>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Russ Hamilton <behamilton@google.com>

agents/skills/luci-test-results/scripts/luci_triage.py

@@ -103,7 +103,7 @@ def list_failures(build_id, limit=500):
     payload = {
         'invocations': [f'invocations/build-{build_id}'],
         'predicate': {
-            'status': 'UNEXPECTED'
+            'status': 'UNEXPECTED_MASK'
         },
         'pageSize': 1000
     }