Add script to check for intermittent regressions

Fixes #12435

etc/ci/buildbot_steps.yml

@@ -27,6 +27,14 @@ mac-nightly:
   - ./mach package --release
   - ./etc/ci/upload_nightly.sh mac
 
+linux-rel-intermittent:
+  - ./mach build --release
+  - ./etc/ci/check_intermittents.sh
+
+mac-rel-intermittent:
+  - ./mach build --release
+  - ./etc/ci/check_intermittents.sh
+
 linux-dev:
   - ./mach test-tidy --no-progress --all
   - ./mach test-tidy --no-progress --self-test

etc/ci/check_intermittents.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+REPEAT_COUNT=100
+
+for test_type in wpt css; do
+    while read test_name; do
+        echo "  - Checking ${test_name}"
+        ./mach "test-${test_type}" \
+            --release \
+            --log-raw - \
+            --repeat "${REPEAT_COUNT}" \
+            "${test_name}" \
+            > intermittents.log \
+            < /dev/null
+    done < "etc/ci/former_intermittents_${test_type}.txt"
+done
+

etc/ci/former_intermittents_css.txt

@@ -0,0 +1,3 @@
+/css-transforms-1_dev/html/transform-table-007.htm
+/css-transforms-1_dev/html/transform-abspos-002.htm
+/css-transforms-1_dev/html/transform-abspos-007.htm