bug#23560: Sometimes network-stream-tests hang forever

[Glenn Morris]

Package: emacs
Version: 25.1.50

Current master on RHEL 7.2.
Sometimes network-stream-tests hang "forever" (I killed it after 10
minutes of nothing happening).

A similar hang is visible on hydra several times, eg
http://hydra.nixos.org/build/35810871 (it tried 3 times, once it aborted
after 2 hours, from watching the log as it was happening I know it was
in network-stream-tests).

My network-stream-tests.log file says (only):

    Running 10 tests (2016-05-16 17:36:18-0700)
    gnutls-serv: Set static Diffie-Hellman parameters, consider --dhparams.
    HTTP Server listening on IPv4 0.0.0.0 port 44331...bind() failed: Address 
already in use
    HTTP Server listening on IPv6 :: port 44331...bind() failed: Address 
already in use

Examining the process listing shows:

UID        PID  PPID  C STIME TTY      STAT   TIME CMD
gmorris  25251 25187  0 17:36 pts/4    Sl+    0:00 ../src/emacs -batch 
--no-site-file --no-site-lisp -L :. -l ert -l lisp/net/networ
gmorris  25320 25251  0 17:36 pts/11   Ss+    0:00 /usr/bin/gnutls-serv --http 
--x509keyfile data/net/key.pem --x509certfile data/ne