Fix TestPosixSpawn.test_close_file() (GH-8992)

Modify TestPosixSpawn to run Python using -I and -S options.

Disable site module to avoid side effects. For example, on Fedora 28,
if the HOME environment variable is not set, site._getuserbase()
calls pwd.getpwuid() which opens /var/lib/sss/mc/passwd, but then
leaves the file open which makes test_close_file() to fail.