Add failing test reproducing issue #1015.

There is a race condition when connect and bind on a new inproc
endpoint happen "simultaneously" in threads.  Causes the error:
  Assertion failed: ok (ctx.cpp:474)
Mark failing test for issue #1015 as XFAIL
Fix race condition in connecting inproc sockets
Merge branch 'inproc_simult' of https://github.com/jemc/libzmq
Merge branch 'issue_1015' of https://github.com/hurtonm/libzmq