test_configdsl: fix test using pipe command (#5159)

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

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

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>'
```

This path will save the command's result in temporary files and use them
instead due to avoid the error.


**Docs Changes**:
N/A

**Release Note**: 
N/A

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

Author: Watson1978

test/test_configdsl.rb

@@ -1,6 +1,7 @@
 require_relative 'helper'
 require 'fluent/config/dsl'
 require 'fluent/test'
+require 'tempfile'
 
 class ConfigDSLTest < Test::Unit::TestCase
   # TEST_CONFIG1 = %[
@@ -107,8 +108,12 @@ def test_config_error
 
   def test_with_ruby_keyword
     uname_string = `uname -a`
+    tmpfile = Tempfile.create('fluentd-test')
+    tmpfile.write(uname_string)
+    tmpfile.close
+
     root1 = Fluent::Config::DSL::Parser.parse(<<DSL)
-uname_str = ruby.open('|uname -a'){|out| out.read}
+uname_str = ruby.open("#{tmpfile.path}"){|out| out.read}
 source {
   uname uname_str
 }
@@ -144,5 +149,7 @@ def test_with_ruby_keyword
 }
 DSL
     assert_raise (NoMethodError) { Fluent::Config::DSL::Parser.parse(conf3) }
+  ensure
+    File.delete(tmpfile.path)
   end
 end