fix: create tmp/tests when the test package loads

Most modules write under tmp/, which a fresh clone lacks, and only three
created it on import. A module run alone failed with FileNotFoundError on
a temporary directory.

tests/__init__.py

@@ -4,10 +4,19 @@ The CI job sets RETROBIOS_OFFLINE, but a local run of the documented command
 did not, and e2e fixtures queried the large-files release and wrote its
 answers into the shared .cache/large. A test that serves its own assets
 clears the variable for itself.
+
+Scratch files go under the repository's tmp/, which git does not track: a
+fresh clone has none, and a module run alone raised FileNotFoundError on a
+temporary directory instead of naming the missing parent.
 """
 
 from __future__ import annotations
 
 import os
+from pathlib import Path
 
 os.environ.setdefault("RETROBIOS_OFFLINE", "1")
+
+(Path(__file__).resolve().parent.parent / "tmp" / "tests").mkdir(
+    parents=True, exist_ok=True
+)