This makes stderr and exit callbacks work for rpc jobs

Temporary change to avoid frequent hangs on Travis macOS/OSX builds.
Hang does not occur on Quickbuild OSX (Yosemite) build.
Reverting e9061117a5b8f195c3f26a5cb94e18ddd7752d86 avoids the hang, but causes
more serious regressions on many more systems.

Note that the job_spec hang only happens with the gcc-4.9 Travis OSX build.

References #5002
References #5029

It appears that used msgpack library is not able to parse back message created 
by msgpack_rpc_from_object() if nesting level is too high, so log_server_msg now 
cares about msgpack_unpack_next() return value. Also error message from 
server_notifications_spec.lua is not readable if something is wrong (though at 
least now it does not crash when parsing deeply nested structures).

log_server_msg() in the test reports

    [msgpack-rpc] nvim -> client(1) [error]        "parse error"

This removes some stack overflows in new test regarding deeply nested variables.
Now in place of crashing vim_to_object/msgpack_rpc_from_object/etc it crashes
clear_tv with stack overflow.

This ought to prevent stack overflow, but I do not see this actually working:
*lua* code crashes with stack overflow when trying to deserialize msgpack from
Neovim, Neovim is fine even if nesting level is increased 100x (though test
becomes very slow); not sure how recursive function may survive this. So it
looks like there are currently only two positive effects:

1. NULL lists are returned as empty (#4596).
2. Functional tests are slightly more fast. Very slightly. Checked for Release
   build for test/functional/eval tests because benchmarking of debug mode is
   not very useful.

It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.

For that, make luatest ignore the preload.lua files.