Backport(v1.16): test: use IO#popen instead of Kernel#open (#5182)

Backport #4369

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

N/A

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

open("|command") spawns command and create pipes for processing
pipeline.

This feature of Kernel#open is deprecated, replace it with IO#popen.

It fixes the following warning:

  warning: Calling Kernel#open with a leading '|' is deprecated and will
  be removed in Ruby 4.0; use IO.popen instead

**Docs Changes**:

N/A

**Release Note**: 

N/A

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

Author: Watson1978

test/plugin_helper/test_child_process.rb

@@ -569,7 +569,7 @@ def configure(conf)
   unless Fluent.windows?
     test 'can specify subprocess name' do
       io = IO.popen([["cat", "caaaaaaaaaaat"], '-'])
-      process_naming_enabled = (open("|ps opid,cmd"){|_io| _io.readlines }.count{|line| line.include?("caaaaaaaaaaat") } > 0)
+      process_naming_enabled = (IO.popen(["ps", "opid,cmd"]){|_io| _io.readlines }.count{|line| line.include?("caaaaaaaaaaat") } > 0)
       Process.kill(:TERM, io.pid) rescue nil
       io.close rescue nil
 
@@ -586,7 +586,7 @@ def configure(conf)
           m.lock
           ran = true
           pids << @d.child_process_id
-          proc_lines += open("|ps opid,cmd"){|_io| _io.readlines }
+          proc_lines += IO.popen(["ps", "opid,cmd"]){|_io| _io.readlines }
           m.unlock
           readio.read
         end
@@ -645,8 +645,8 @@ def configure(conf)
   unless Fluent.windows?
     test 'can change working directory' do
       # check my real /tmp directory (for mac)
-      cmd = %[|ruby -e 'Dir.chdir("/tmp"); puts Dir.pwd']
-      mytmpdir = open(cmd){|io| io.read.chomp }
+      cmd = ['ruby', '-e', 'Dir.chdir("/tmp"); puts Dir.pwd']
+      mytmpdir = IO.popen(cmd){|io| io.read.chomp }
 
       m = Mutex.new
       str = nil