Show filesystem encoding in failure message

`test_cjk_in_path` failed on mswinci yet.

https://rubyci.org/logs/mswinci.japaneast.cloudapp.azure.com/vc12-x64/ruby-master/log/20200616T192319Z.fail.html.gz
```
  1) Failure:
WEBrick::TestFileHandler#test_cjk_in_path [D:/tmp/mswin-build20200617-120024-1brdn58/ruby/test/webrick/utils.rb:72]:
exceptions on 2 threads:
webrick log start:
  [2020-06-17 06:12:53] ERROR `/あ.txt' not found.

webrick log end.
<"200"> expected but was
<"404">.
---
<[]> expected but was
<["[2020-06-17 06:12:53] ERROR `/\xE3\x81\x82.txt' not found.\n"]>.
```

test/webrick/test_filehandler.rb

@@ -295,7 +295,7 @@ def test_cjk_in_path
       TestWEBrick.start_httpserver(config) do |server, addr, port, log|
         http = Net::HTTP.new(addr, port)
         req = Net::HTTP::Get.new("/%E3%81%82.txt")
-        http.request(req){|res| assert_equal("200", res.code, log.call) }
+        http.request(req){|res| assert_equal("200", res.code, log.call + "\nFilesystem encoding is #{Encoding.find('filesystem')}") }
       end
     end
   end