Oh well, it's really hard to test the tests in my working copy because it depends on a lot of factors: the current working directory, is Python installed or not, the OS, etc. The test introduced in r78835 was incorrect (only the test, the fix is correct): r78836 fixes the test for Windows (where sys.executable is correct even if argv[0] is set to a non existent program name, I don't understand how) and r78837 fixes the test for Ubuntu.
r78837 is a temporary fix to workaround #7774 bug: it's the same hack used in test_executable_with_cwd() from test_subprocess.