Add tests for pretty printing

Author: simsurace

test/directed.jl

@@ -239,3 +239,7 @@ for layer in all_layers_d
         end
     end
 end
+
+@testset "Printing MultilayerDiGraph" begin
+    @test_nowarn @show multilayergraph
+end

test/undirected.jl

@@ -244,3 +244,7 @@ for layer in all_layers_u
         end
     end
 end
+
+@testset "Printing MultilayerGraph" begin
+    @test_nowarn @show multilayergraph
+end