test_dsl: fix test using pipe command

[Watson1978]

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

[daipom]

Thanks!