tests: fix edge case in checkpoint test

When running Python tests locally multiple times, the checkpoint
load tests may succeed even if the corresponding checkpoint save
tests failed. Old checkpoint files are now garbage collected.

testsuite/python/save_checkpoint.py

@@ -48,6 +48,12 @@
 checkpoint = espressomd.checkpointing.Checkpoint(
     checkpoint_id=idx, checkpoint_path="@CMAKE_CURRENT_BINARY_DIR@")
 
+# cleanup old checkpoint files
+if checkpoint.has_checkpoints():
+    for filepath in os.listdir(checkpoint.checkpoint_dir):
+        if filepath.endswith((".checkpoint", ".cpt")):
+            os.remove(os.path.join(checkpoint.checkpoint_dir, filepath))
+
 LB_implementation = None
 if 'LB.CPU' in modes:
     LB_implementation = espressomd.lb.LBFluid