TaskLocalRNG: test that copy handles the splitmix state

This adds a test for #51332.

stdlib/Random/test/runtests.jl

@@ -1065,3 +1065,26 @@ end
         end
     end
 end
+
+@testset "TaskLocalRNG: copy and copy! handle the splitmix state" begin
+    seeds = rand(RandomDevice(), UInt64, 5)
+    for seed in seeds
+        Random.seed!(seed)
+        rng1 = copy(TaskLocalRNG())
+        x = fetch(@async rand(UInt64))
+        rng2 = copy(TaskLocalRNG())
+        y = fetch(@async rand(UInt64))
+        rng3 = copy(TaskLocalRNG())
+        @test x != y
+        @test rng1 != rng2
+        Random.seed!(seed)
+        @test TaskLocalRNG() == rng1
+        @test x == fetch(@async rand(UInt64))
+        @test TaskLocalRNG() == rng2
+        # this should be a no-op:
+        copy!(TaskLocalRNG(), copy(TaskLocalRNG()))
+        @test TaskLocalRNG() == rng2
+        @test y == fetch(@async rand(UInt64))
+        @test TaskLocalRNG() == rng3
+    end
+end