test_dsl: fix test using pipe command (#5160)

**Which issue(s) this PR fixes**: 
Fixes #5148

**What this PR does / why we need it**: 
Related to #5159
I overlooked the fact that there were also tests using pipe commands in
another file.

Since Ruby 4.0, When we give pipe command in `Kernel.open`, it causes an
error.

```
$ ruby -w -v -e "Kernel.open('| uname -a'){|out| p out.read }"
ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x86_64-linux]
-e:1: warning: Calling Kernel#open with a leading '|' is deprecated and will be removed in Ruby 4.0; use IO.popen instead
"Linux UM890 6.17.1-0-MANJARO #1 SMP PREEMPT_DYNAMIC Tue, 07 Oct 2025 07:57:35 +0000 x86_64 GNU/Linux\n"

$ ruby -w -v -e "Kernel.open('| uname -a'){|out| p out.read }"
ruby 4.0.0dev (2025-11-20T00:03:23Z master 167c3dbaa0) +PRISM [x86_64-linux]
-e:1:in 'File#initialize': No such file or directory @ rb_sysopen - | uname -a (Errno::ENOENT)
	from -e:1:in 'Kernel.open'
	from -e:1:in '<main>'
```

**Docs Changes**:
N/A

**Release Note**: 
N/A

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

test/config/test_dsl.rb

@@ -1,6 +1,7 @@
 require_relative '../helper'
 require 'fluent/config/element'
 require "fluent/config/dsl"
+require 'tempfile'
 
 TMP_DIR = File.dirname(__FILE__) + "/tmp/config_dsl#{ENV['TEST_ENV_NUMBER']}"
 def write_config(path, data)
@@ -358,9 +359,13 @@ def setup
       sub_test_case '.parse' do
         test 'can get result of Kernel.open() by ruby.open()' do
           uname_string = `uname -a`
+          tmpfile = Tempfile.create('fluentd-test')
+          tmpfile.write(uname_string)
+          tmpfile.close
+
           root = Fluent::Config::DSL::Parser.parse(<<DSL)
 worker {
-  uname_str = ruby.open('|uname -a'){|out| out.read}
+  uname_str = ruby.open("#{tmpfile.path}"){|out| out.read}
   source {
     uname uname_str
   }
@@ -372,6 +377,8 @@ def setup
           assert_equal('source', source.name)
           assert_equal(1, source.keys.size)
           assert_equal(uname_string, source['uname'])
+        ensure
+          File.delete(tmpfile.path)
         end
 
         test 'accepts ruby keyword with block, which allow to use methods included from ::Kernel' do