test: fix box/iproto_stress sporadic failure

This patch fixes the following test failure:

 | --- box/iproto_stress.result	Tue Dec 25 09:56:54 2018
 | +++ box/iproto_stress.reject	Tue Dec 25 10:12:22 2018
 | @@ -80,7 +80,7 @@
 |  ...
 |  n_workers -- 0
 |  ---
 | -- 0
 | +- 340
 |  ...
 |  n_errors -- 0
 |  ---
 | @@ -93,5 +93,3 @@
 |  ---
 |  ...
 |  box.cfg{net_msg_max = net_msg_max}
 | ----
 | -...

The problem is the test is quite cpu intensive so if the host is heavily
loaded (as it is often the case when tests are run on Travis CI), it may
take a few minutes to complete, while the timeout is set to 10 seconds.

To fix it, let's
 - Increase the timeout up to 60 seconds and use test_run.wait_cond
   instead of a homebrew loop.
 - Decrease the number of fibers from 400 down to 100 and adjust
   box.cfg.net_msg_max respectively.

Closes #3911