Add test for warning output

test/mix/tasks/dialyzer_test.exs

@@ -67,4 +67,12 @@ defmodule Mix.Tasks.DialyzerTest do
  assert result =~
  ~r/Total errors: ., Skipped: ., Unnecessary Skips: .\ndone \(passed successfully\)\n/
  end
+
+ @tag :output_tests
+ test "Warning is printed when unknown format is requested" do
+ args = ["dialyzer", "--format", "foo"]
+ env = [{"MIX_ENV", "prod"}]
+ {result, 0} = System.cmd("mix", args, env: env)
+ assert result =~ "Unrecognized formatter foo received. Known formatters are dialyzer, dialyxir, github, ignore_file, ignore_file_string, raw, and short. Falling back to dialyxir."
+ end
 end