test_box: avoid failure with --program-suffix (ruby#15734)

Author: sorah

test/ruby/test_box.rb

@@ -1,10 +1,11 @@
 # frozen_string_literal: true
 
 require 'test/unit'
+require 'rbconfig'
 
 class TestBox < Test::Unit::TestCase
   EXPERIMENTAL_WARNING_LINE_PATTERNS = [
-    /ruby(\.exe)?: warning: Ruby::Box is experimental, and the behavior may change in the future!/,
+    /#{RbConfig::CONFIG["ruby_install_name"] || "ruby"}(\.exe)?: warning: Ruby::Box is experimental, and the behavior may change in the future!/,
     %r{See https://docs.ruby-lang.org/en/(master|\d\.\d)/Ruby/Box.html for known issues, etc.}
   ]
   ENV_ENABLE_BOX = {'RUBY_BOX' => '1', 'TEST_DIR' => __dir__}