[core] Fix gdb check for CI

scripts/run_tests.sh

@@ -24,7 +24,7 @@ if [ ! -d "test/node_modules/express" ]; then
     (cd test; npm install express@4.11.1)
 fi
 
-if which -s 'gdb'; then
+if command -v gdb >/dev/null 2>&1; then
     gdb -batch -return-child-result -ex 'set print thread-events off' \
         -ex 'run' -ex 'thread apply all bt' --args ${CMD} ;
 else