tests: fix flacky test_running_base_functionality

The integration test `test_running_cluster::test_running_base_functionality`
fails sometimes with an failed assertion of the same PID after restart.

After the patch waiting for instances stop was added to make sure that
cluster is down before it will start again and will check for PID.

Closes #972

test/integration/running/test_running_cluster.py

@@ -7,7 +7,8 @@
 import yaml
 
 from utils import (control_socket, extract_status, get_tarantool_version,
-                   run_command_and_get_output, run_path, wait_file)
+                   pid_file, run_command_and_get_output, run_path, wait_file,
+                   wait_instance_stop)
 
 tarantool_major_version, tarantool_minor_version = get_tarantool_version()
 
@@ -99,6 +100,11 @@ def test_running_base_functionality(tt_cmd, tmpdir_with_cfg):
         )
         start_output = instance_process.stdout.read()
 
+        # Need to wait for all instances to stop before start.
+        for instance in instances:
+            pid_path = os.path.join(tmpdir, app_name, run_path, instance, pid_file)
+            wait_instance_stop(pid_path)
+
         for inst in instances:
             assert f"Starting an instance [{app_name}:{inst}]" in start_output