btest-bg-run-helper: Ignore SIGTERM to self during cleanup

The kill 0 invocation would kill the btest-bg-run-helper itself,
causing the subsequent logic to not be executed, resulting in
runaway processes on the system.

btest-bg-run-helper

@@ -3,9 +3,21 @@
 # Internal helper for btest-bg-run.
 
 cleanup() {
+    # Ignore SIGTERM during cleanup to prevent terminating
+    # this process when sending signals to the process group.
+    trap true SIGTERM
+
     if [ ! -e .exitcode ]; then
         echo 15 >.exitcode
-        kill 0 &>/dev/null
+
+        # Send SIGTERM to all processes of the process group.
+        # We do not use pid 0 here, as the btest-setsid helper
+        # may have failed and we don't want to send signals to
+        # processes in a process group we never created.
+        #
+        # This will terminate any well-behaved background commands
+        # that were spawned by the program under test.
+        kill SIGTERM -${BASHPID} &>/dev/null
 
         if [ -n "$pid" ]; then
             kill -0 "$pid" &>/dev/null && kill "$pid"