Revert "remove Combinatorics as a test dep, as it's not needed for this PR"

This reverts commit 071bebf.

Author: torfjelde

test/Project.toml

@@ -2,6 +2,7 @@
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"
 AbstractPPL = "7a57a42e-76ec-4ea3-a279-07e840d6d9cf"
 Bijectors = "76274a88-744f-5084-9051-94815aaf08c4"
+Combinatorics = "861a8166-3701-5b0c-9a16-15d98fcdc6aa"
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"
 Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 Distributions = "31c24e10-a181-5473-b8eb-7969acd0382f"
@@ -26,6 +27,7 @@ Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 AbstractMCMC = "5"
 AbstractPPL = "0.7"
 Bijectors = "0.13"
+Combinatorics = "1"
 Compat = "4.3.0"
 Distributions = "0.25"
 DistributionsAD = "0.6.3"

test/runtests.jl

@@ -20,6 +20,8 @@ using Random
 using Serialization
 using Test
 
+using Combinatorics: combinations
+
 using DynamicPPL: getargs_dottilde, getargs_tilde, Selector
 
 const DIRECTORY_DynamicPPL = dirname(dirname(pathof(DynamicPPL)))