test: Add workaround for FreeBSD libedit vs readline issue

If stdout is not a terminal Python executes
    rl_variable_bind ("enable-meta-key", "off");

This produces a warning with FreeBSD's libedit because the
enable-meta-key variable is unknown.  Not an issue on Apple because
cpython commit f0ab6f9f0603 added a #ifndef __APPLE__ around the call.
See http://bugs.python.org/issue19884 for more information.

For now we just discard the warning output to get the tests working
again on FreeBSD.

llvm-svn: 226037