[3.8] bpo-38456: Use /bin/true in test_subprocess (GH-16737)

* [3.8] bpo-38456: Use /bin/true in test_subprocess (GH-16736)

Instead of sys.executable, "-c", "pass" or "import sys; sys.exit(0)"
use /bin/true when it is available.  On a reasonable machine this
shaves up to two seconds wall time off the otherwise ~40sec execution
on a --with-pydebug build.  It should be more notable on many
buildbots or overloaded slower I/O systems (CI, etc)..
(cherry picked from commit 67b93f80c764bca01c81c989d74a99df208bea4d)

* Handle when there is no 'true' command

backport of 46113e0cf32748f66cf64cd633984d143b433cd1 by Pablo Galindo.
1 file changed