Set ASCII-8BIT encoding.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47979 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: akr

test/-ext-/bug_reporter/test_bug_reporter.rb

@@ -18,6 +18,7 @@ def test_bug_reporter_add
             "-C", tmpdir]
     stdin = "register_sample_bug_reporter(12345); Process.kill :SEGV, $$; sleep"
     _, stderr, status = EnvUtil.invoke_ruby(args, stdin, false, true)
+    stderr.force_encoding("ASCII-8BIT")
     assert_pattern_list(expected_stderr, stderr)
   ensure
     FileUtils.rm_rf(tmpdir) if tmpdir