Fix test failure with -DNDEBUG.

The elf/tst-dlopen-aout.c test uses asserts to verify properties of the
test execution.  Instead of using assert it should use xpthread_create
and xpthread_join to catch errors starting the threads and fail the
test.  This shows up in Fedora 28 when building for i686-pc-linux-gnu
and using gcc 8.1.1.

Tested on i686, and fixes a check failure with -DNDEBUG.

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