test/zlib/test_zlib.rb: Set binmode to test output file

Seems like the test `TestZlibGzipFile#test_gzip_reader_zcat` fails when
the timestamp has `\n\n`.

https://ci.appveyor.com/project/ruby/ruby/builds/38597932
```
  1) Error:
TestZlibGzipFile#test_gzip_reader_zcat:
Zlib::DataError: invalid distance too far back
    C:/projects/ruby/test/zlib/test_zlib.rb:522:in `initialize'
    C:/projects/ruby/test/zlib/test_zlib.rb:522:in `new'
    C:/projects/ruby/test/zlib/test_zlib.rb:522:in `zcat'
    C:/projects/ruby/test/zlib/test_zlib.rb:522:in `block (2 levels) in test_gzip_reader_zcat'
    C:/projects/ruby/test/zlib/test_zlib.rb:521:in `open'
    C:/projects/ruby/test/zlib/test_zlib.rb:521:in `block in test_gzip_reader_zcat'
    C:/projects/ruby/lib/tempfile.rb:358:in `create'
    C:/projects/ruby/test/zlib/test_zlib.rb:510:in `test_gzip_reader_zcat'
```

The test time is around 2021-04-08 04:40 +0900. Maybe the following time
should trigger the bug.

```
irb(main):001:0> Time.at(1617824266)
=> 2021-04-08 04:37:46 +0900
irb(main):002:0> [1617824266].pack("V")
=> "\n\nn`"
```

test/zlib/test_zlib.rb

@@ -508,6 +508,7 @@ if defined? Zlib
   class TestZlibGzipFile < Test::Unit::TestCase
     def test_gzip_reader_zcat
       Tempfile.create("test_zlib_gzip_file_to_io") {|t|
+        t.binmode
         gz = Zlib::GzipWriter.new(t)
         gz.print("foo")
         gz.close