Backport(v1.16) test_out_file: fix file open mode for binary file (#4798

)

**Which issue(s) this PR fixes**: 

Backport #4728 

**What this PR does / why we need it**: 

Fix the file open mode so that gz files can be read properly.

**Docs Changes**:

**Release Note**:

Signed-off-by: Shizuo Fujita <[email protected]>
Signed-off-by: Kentaro Hayashi <[email protected]>
Co-authored-by: Shizuo Fujita <[email protected]>

test/plugin/test_out_file.rb

@@ -334,7 +334,7 @@ def create_driver(conf = CONFIG, opts = {})
       assert_equal r5, d.formatted[4]
 
       read_gunzip = ->(path){
-        File.open(path){ |fio|
+        File.open(path, 'rb'){ |fio|
           Zlib::GzipReader.new(StringIO.new(fio.read)).read
         }
       }