[rubygems/rubygems] Require open3 before using it

Otherwise if this test file is run in isolation, this test will fail.

https://github.com/rubygems/rubygems/commit/79da003948

test/rubygems/test_gem.rb

@@ -305,6 +305,7 @@ class TestGem < Gem::TestCase
 
     install_specs a1
 
+    require "open3"
     output, status = Open3.capture2e(
       { "GEM_HOME" => Gem.paths.home, "DEBUG_RESOLVER" => "1" },
       *ruby_with_rubygems_in_load_path, "-e", "\"Gem.activate_bin_path('a', 'exec', '>= 0')\""