Set tmp_path_retention_policy = 'failed' (#1413)

Author: NicolasHug

pyproject.toml

@@ -54,4 +54,8 @@ norecursedirs = ["third-party-interface"]
 # the CI, where we definitely want the 'slow' tests to run.
 addopts = "-v -m 'not slow'"
 
+# Tells pytest to keep around temporary paths (via tmp_path fixture) for
+# *failed tests* only. Otherwise, the /tmp folder can grow too large.
+tmp_path_retention_policy = "failed"
+
 testpaths = ["test"]