Use gdb to get backtraces

Core dumps don't work on Travis docker containers.

scripts/run_tests.sh

@@ -24,21 +24,5 @@ if [ ! -d "test/node_modules/express" ]; then
     (cd test; npm install express@4.11.1)
 fi
 
-RESULT=0
-${CMD} "$@" || RESULT=$?
-
-if [[ ${RESULT} != 0 ]]; then
-    echo "The program crashed with exit code ${RESULT}. We're now trying to output the core dump."
-fi
-
-# output core dump if we got one
-for DUMP in $(find ./ -maxdepth 1 -name 'core*' -print); do
-    gdb ${CMD} ${DUMP} -ex "thread apply all bt" -ex "set pagination 0" -batch
-    rm -rf ${DUMP}
-done
-
-# now we should present travis with the original
-# error code so the run cleanly stops
-if [[ ${RESULT} != 0 ]]; then
-    exit $RESULT
-fi
+gdb -batch -return-child-result -ex 'set print thread-events off' \
+    -ex 'run' -ex 'thread apply all bt' ${CMD}