.credo.exs: Add test directory as well

.credo.exs

@@ -19,7 +19,7 @@
         #
         # You can give explicit globs or simply directories.
         # In the latter case `**/*.{ex,exs}` will be used.
-        included: ["lib/", "src/", "web/", "apps/"],
+        included: ["lib/", "src/", "web/", "apps/", "test/"],
         excluded: [~r"/_build/", ~r"/deps/"]
       },
       #