Given that we're already monkeypatching here, use this to also

fake out os.path.exists to get the old behaviour for the test. Add
a new assert to test the new behaviour first.

tests/serialize/test_filename.py

@@ -1,15 +1,30 @@
 from __future__ import annotations
 
 import os
+import uuid
 
 from dxtbx.serialize.filename import resolve_path
 
 
 def test_resolve_path(monkeypatch):
+    # Resolve path
+    def alwaystrue(path):
+        return True
+
+    # Set an environment variable
     monkeypatch.setenv("HELLO_WORLD", "EXPANDED")
-    new_path = os.path.join("~", "$HELLO_WORLD", "path")
+
+    # First try a path that does not exist
+    filename = str(uuid.uuid4())
+    new_path = os.path.join("~", "$HELLO_WORLD", filename)
     path = resolve_path(new_path)
-    assert path == os.path.join(os.path.expanduser("~"), "EXPANDED", "path")
-    new_path = os.path.join("$HELLO_WORLD", "path")
+    assert path == new_path
+
+    # Now pretend the path exists
+    monkeypatch.setattr(os.path, "exists", alwaystrue)
+    path = resolve_path(new_path)
+    assert path == os.path.join(os.path.expanduser("~"), "EXPANDED", filename)
+
+    new_path = os.path.join("$HELLO_WORLD", filename)
     path = resolve_path(new_path)
-    assert path == os.path.abspath(os.path.join("EXPANDED", "path"))
+    assert path == os.path.abspath(os.path.join("EXPANDED", filename))