Actually, I was still working on this.  I had assigned it to myself.

+        p = subprocess.Popen(["nonexistent","-c",'import sys; sys.exit(42)'],
+                            executable=sys.executable, cwd=python_dir)

The test for the executable argument should really test the executable argument independent of the cwd argument.  Also, it would be better if the test uses a valid args[0] to demonstrate precedence.  I had prepared a test but did not upload it yet, because I also wanted to add an accompanying clarification to the docs which I am still preparing.  Reopening.