Fix failure file dump when output has invalid UTF8

Previously, any invalid UTF-8 in test output would prevent the entire
JSON failure file from being dumped, because the Ruby JSON library
validates that its output is UTF-8.

This commit aims to fix this issue by encoding the output as UTF-8
before it gets to JSON so that the file can always be dumped.

Author: skipkayhil

ruby/lib/minitest/queue/failure_formatter.rb

@@ -13,11 +13,9 @@ def initialize(test)
       end
 
       def to_s
-        [
-          header,
-          body,
-          "\n"
-        ].flatten.compact.join("\n")
+        s = +"#{header}\n#{body}\n\n"
+        s.encode!(Encoding::UTF_8, invalid: :replace, undef: :replace)
+        s
       end
 
       def to_h

ruby/test/minitest/queue/failure_formatter_test.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+module Minitest::Queue
+  class FailureFormatterTest < Minitest::Test
+    include ReporterTestHelper
+
+    def test_failure_formatter_to_h_can_be_json_dumped
+      test = result('test_json', failure: "\xD6".b)
+
+      formatter = FailureFormatter.new(test)
+
+      assert JSON.dump(formatter.to_h)
+    end
+  end
+end