Distributed test suite: mark another test as thread-unsafe (#42941)

(cherry picked from commit 7c9d126)

Author: DilumAluthge

stdlib/Distributed/test/distributed_exec.jl

@@ -293,7 +293,9 @@ let wid1 = workers()[1],
     end
     finalize(rr) # finalize locally
     yield() # flush gc msgs
-    @test remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid) == true
+    if include_thread_unsafe()
+        @test remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid) == true
+    end
     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 remotecall_fetch(k -> haskey(Distributed.PGRP.refs, k), wid1, rrid) == false