fix thread unsafe test

Author: IanButterworth

test/distributed_exec.jl

@@ -302,8 +302,8 @@ let wid1 = workers()[1],
     finalize(rr) # finalize locally
     yield() # flush gc msgs
     if include_thread_unsafe_tests()
-        @test remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid) == true
     end
+        @test timedwait(() -> remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid), 10) === :ok
     remotecall_fetch(r -> (finalize(take!(r)); yield(); nothing), wid2, fstore) # finalize remotely
     sleep(0.5) # to ensure that wid2 messages have been executed on wid1
     @test poll_while(() -> remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid))