add some new tests

Author: sunxd3

test/abstractmcmc.jl

@@ -27,6 +27,10 @@ using Statistics: mean
         θ = AbstractMCMC.getparams(s)
         @test θ == t.z.θ
         @test AbstractMCMC.setparams!!(s, θ) == s
+
+        new_θ = randn(rng, 2)
+        new_state = AbstractMCMC.setparams!!(s, new_θ)
+        @test AbstractMCMC.getparams(new_state) == new_θ
     end
 
     samples_nuts = AbstractMCMC.sample(