Add tests for derangements

Two tests are currently broken because `derangements` returns an empty
vector of the wrong type `Int[]` instead of `[Int[]]`.

Author: FedericoStra

test/permutations.jl

@@ -66,6 +66,16 @@ end
     @test collect(multiset_permutations("", -1)) == Any[]
     @test length(multiset_permutations("aaaaaaaaaaaaaaaaaaaaab", 21)) == 22
 
+    # derangements
+    @test length(collect(derangements(1:4))) == 9
+    @test length(collect(derangements(1:8))) == derangement(8) == 14833
+    @test collect(derangements(Int[])) == [Int[]]
+    @test_broken collect(derangements([1, 1])) == [Int[]] # currently returns Int[]
+    @test_broken collect(derangements([1, 1, 2])) == [Int[]] # currently returns Int[]
+    @test collect(derangements([1, 1, 2, 2])) == [[2, 2, 1, 1]]
+    @test map(join, derangements("aabbc")) == ["bbaca", "bbcaa", "bcaab", "cbaab"]
+    @test map(join, derangements("aaabbbc")) == ["bbbaaca", "bbbacaa", "bbbcaaa", "bbcaaab", "bcbaaab", "cbbaaab"]
+
     #nthperm!
     for n = 0:7, k = 1:factorial(n)
         p = nthperm!([1:n;], k)