Find errors in dig output in system tests

Facilitate faster system test failure identification and debugging by
checking any dig outputs for errors, which are typically indicative of
CI runner network / load issues.

.gitlab-ci.yml

@@ -321,6 +321,7 @@ stages:
     - REALSOURCEDIR="$PWD"
     - test -n "${OUT_OF_TREE_WORKSPACE}" && cd "${OUT_OF_TREE_WORKSPACE}"
     - cat bin/tests/system/test-suite.log
+    - find bin/tests/system -name "*dig.*" | xargs grep "error"
     - *find_python
     - >
       "$PYTHON" "$REALSOURCEDIR"/bin/tests/convert-trs-to-junit.py . > "$CI_PROJECT_DIR"/junit.xml
@@ -343,6 +344,7 @@ stages:
   <<: *system_test_common
   after_script:
     - cat bin/tests/system/test-suite.log
+    - find bin/tests/system -name "*dig.*" | xargs grep "error"
     - *find_python
     - *parse_tsan
     - >