Put -W:no-experimental after whatever RUBYOPT already carries

The last -W switch wins, so prepending it left an inherited -w or -W2 in
control and the suppression quietly did nothing.  `make test-all
RUBYOPT=-w` reproduces it: under `RUBY_BOX=1` test_open3.rb and
test_pty.rb stay at 12 and 4 failures, and reach 0 once the switch moves
behind the inherited options and in front of the bare "-".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

spec/default.mspec

@@ -3,8 +3,11 @@ $VERBOSE = false
 if (opt = ENV["RUBYOPT"]) and (opt = opt.dup).sub!(/(?:\A|\s)-w(?=\z|\s)/, '')
   ENV["RUBYOPT"] = opt
 end
-# The specs assert the output of the ruby processes they spawn verbatim.
-ENV["RUBYOPT"] = (["-W:no-experimental"] | ENV["RUBYOPT"].to_s.split).join(" ")
+# The specs assert the output of the ruby processes they spawn verbatim.  See
+# tool/test/init.rb for why the switch goes where it does.
+rubyopt = ENV["RUBYOPT"].to_s.split - ["-W:no-experimental"]
+rubyopt.insert(rubyopt.index("-") || rubyopt.size, "-W:no-experimental")
+ENV["RUBYOPT"] = rubyopt.join(" ")
 
 # Enable constant leak checks by ruby/mspec
 ENV["CHECK_CONSTANT_LEAKS"] ||= "true"

tool/test/init.rb

@@ -14,9 +14,12 @@ ENV.delete("RUBY_CODESIGN")
 
 Warning[:experimental] = false
 # The tests assert the output of the ruby processes they spawn verbatim, so the
-# children need this too.  It has to precede the bare "-" that common.mk puts in
-# RUBYOPT, which ends the option scan.
-ENV["RUBYOPT"] = (["-W:no-experimental"] | ENV["RUBYOPT"].to_s.split).join(" ")
+# children need this too.  The last -W wins, so the switch has to come after
+# whatever RUBYOPT already carries, but before the bare "-" that common.mk puts
+# there, which ends the option scan.
+rubyopt = ENV["RUBYOPT"].to_s.split - ["-W:no-experimental"]
+rubyopt.insert(rubyopt.index("-") || rubyopt.size, "-W:no-experimental")
+ENV["RUBYOPT"] = rubyopt.join(" ")
 
 $LOAD_PATH.unshift File.expand_path("../lib", __dir__)