Run nptl/tst-pthread-getattr in a container

See https://bugzilla.redhat.com/show_bug.cgi?id=1653942



This test depends on the kernel's assignment of memory regions, but
running under ld.so explicitly changes those assignments, sometimes
sufficiently to cause the test to fail (esp with address space
randomization).

The easiest way to "fix" the test, is to run it the way the user would
- without ld.so.  Running it in a container does that.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>