add ambiguity test that Base CI has

Author: IanButterworth

test/runtests.jl

@@ -1,6 +1,10 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
 using Test
+using Distributed
+
+allowed_undefineds = Set([GlobalRef(Distributed, :cluster_manager)])
+@test isempty(detect_ambiguities(Distributed; allowed_undefineds))
 
 # Run the distributed test outside of the main driver since it needs its own
 # set of dedicated workers.