add tests from ForwardDiff

Author: mcabbott

test/Project.toml

@@ -2,6 +2,7 @@
 ChainRules = "082447d4-558c-5d27-93f4-14fc19e9eca2"
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 Combinatorics = "861a8166-3701-5b0c-9a16-15d98fcdc6aa"
+DiffTests = "de460e47-3fe3-5279-bb4a-814414816d5d"
 Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 FiniteDifferences = "26cc04aa-876d-5657-8c51-4c34ba976000"
 ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
@@ -19,6 +20,7 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 ChainRules = "1.5"
 ChainRulesCore = "1.2"
 Combinatorics = "1"
+DiffTests = "0.1.1"
 StaticArrays = "1"
 StatsBase = "0.33"
 StructArrays = "0.6"