Fix a race condition in the tests

Quick explanation of what the `GC tests for RemoteChannels` test does:
1. Create `RemoteChannel`s `rr` and `fstore` on worker 1 and worker 2
   respectively from the master process. At this point only the master process
   knows about `rr` and `fstore`.
2. Master process calls `put!(fstore, rr)`, i.e. we remotecall worker 2 and put
   `rr` (which is owned worker 1 but is currently only known about by the
   master) into `fstore`.
3. Remotecall into worker 1 and check that it knows about `rr`.

Step 3 should succeed despite us never previously explicitly communicating with
worker 1, because `serialize(::ClusterSerializer, ::RemoteChannel)` will send a
message to the owner of the `RemoteChannel` to inform them of its existence (see
`send_add_client()`). This happens asynchronously in step 2, and on rare
occasions worker 1 would not process that message before step 3, causing the
test to fail.

Now we give the check 10s to succeed.

Author: JamesWrigley

test/distributed_exec.jl

@@ -295,7 +295,9 @@ let wid1 = workers()[1],
 
     put!(fstore, rr)
     if include_thread_unsafe_tests()
-        @test remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid) == true
+        # timedwait() is necessary because wid1 is asynchronously informed of
+        # the existence of rr/rrid through the call to `put!(fstore, rr)`.
+        @test timedwait(() -> remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid), 10) === :ok
     end
     finalize(rr) # finalize locally
     yield() # flush gc msgs