Fix test failures on Mac OS X

By default Mac OS X doesn't allow the process to use a lot of
file descriptors, so lets use fewer in the test case on these
limited os's..

Change-Id: I3627a0463aa634af9dca36ec1892e4e2e349a08c
Reviewed-on: http://review.couchbase.org/27706
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>