Ignore SIGCHLD in test setup script

SIGCHLD is expected when a subprocess ends, and is not indicative of a
failure. So, when trapping on signals to detect issues, do not trap on
this signal.

This has become important with Bash 5, as it has begun propagating this
signal up to this level.

Closes #7461.
Closes #7441.
Progress on #7410 when XML splitting is disabled.

PiperOrigin-RevId: 234568715

tools/test/test-setup.sh

@@ -270,6 +270,8 @@ if [[ "${EXPERIMENTAL_SPLIT_XML_GENERATION}" == "1" ]]; then
   trap 'echo "-- Test timed out at $(date +"%F %T %Z") --"' SIGTERM
 else
   for signal in $signals; do
+    # SIGCHLD is expected when a subprocess dies
+    [ "${signal}" = "SIGCHLD" ] && continue
     trap "write_xml_output_file ${signal}" ${signal}
   done
 fi