Format multiline AssertionError message with correct padding

Author: edgurgel

lib/ex_unit/lib/ex_unit/formatter.ex

@@ -171,6 +171,7 @@ defmodule ExUnit.Formatter do
   end
 
   defp format_banner(value, formatter) do
+    value = String.replace(value, "\n", "\n" <> @counter_padding)
     formatter.(:error_info, value)
   end
 

lib/ex_unit/test/ex_unit/formatter_test.exs

@@ -133,4 +133,15 @@ defmodule ExUnit.FormatterTest do
                 6]
     """
   end
+
+  test "formats assertions with message with multiple lines" do
+    message = "Some meaningful error:\nuseful info\nanother useful info"
+    failure = {:error, catch_assertion(assert(false, message)), []}
+    assert format_test_case_failure(case(), failure, 1, :infinity, &formatter/2) =~ """
+      1) Hello: failure on setup_all callback, tests invalidated
+         Some meaningful error:
+         useful info
+         another useful info
+    """
+  end
 end