Fix race condition in workqueue test.
Failure seen at least once in about 150 test runs:
```
Queue with max size of 160 bytes and 3 max items
couch_work_queue_tests:143: should_have_no_items_for_new_queue...ok
**error:{assertEqual_failed,[{module,couch_work_queue_tests},
{line,210},
{expression,"couch_work_queue : item_count ( Q )"},
{expected,3},
{value,2}]}
```
From
```
should_block_producer_on_full_queue_count({Q, Producer, _}) ->
...
produce(Q, Producer, 20, false),
?assertEqual(3, couch_work_queue:item_count(Q)),
```
Race condition is: telling Producer to produce, not waiting
for queue size to be updated, and then immediately checking
item_count.
To make failure occur reliably insert timer:sleep(10) in
producer_loop after ```Parent ! {item, Ref, Item}``` line.
The fix is to wait for queue size to be updated by using
the wait mode for produce function.
COUCHDB-2848
1 file changed
