Fix tests runner script

Author: maksm90

run_tests.sh

@@ -147,7 +147,7 @@ python3 -m venv /tmp/env && source /tmp/env/bin/activate &&
 pip install -r tests/requirements.txt
 set -e #exit virtualenv with error code
 python tests/pg_qs_test_runner.py --port $PGPORT
-if [ -n $USE_TPCDS ]; then
+if [[ -n $USE_TPCDS ]]; then
 	python tests/pg_qs_test_runner.py --port $PGPORT --tpc-ds
 fi
 deactivate