tests.sh: fix --pdb (set SERIAL when --pdb is set) (#1410)

When we optimized tests runs with gnu parallel execution, we broke the
--pdb flag. This commit fixes it and sets the SERIAL flag so that tests
are run one by one when --pdb is passed.

tests/gdb-tests/tests.sh

@@ -32,7 +32,8 @@ while [[ $# -gt 0 ]]; do
     case $1 in
         -p | --pdb)
             USE_PDB=1
-            echo "Will run tests with Python debugger"
+	    SERIAL=1
+            echo "Will run tests in serial and with Python debugger"
             shift
             ;;
         -c | --cov)