Load RDoc before the test sandbox in TestGemRDoc

RDoc 8 depends on rbs, and that dependency stays unresolved when several
rbs versions are installed. It is then resolved on the first require of
rbs, which happens inside Gem::RDoc#setup, after Gem::TestCase#setup has
pointed GEM_HOME at an empty temporary directory, so it raises LoadError.

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

Author: hsbt

test/rubygems/test_gem_rdoc.rb

@@ -4,6 +4,11 @@
 require_relative "helper"
 require "rubygems/rdoc"
 
+# RDoc resolves its own dependencies lazily, on require. Load them here, while
+# the real gem paths are still in effect, because Gem::TestCase#setup points
+# GEM_HOME at an empty temporary directory where they cannot be found.
+Gem::RDoc.load_rdoc if defined?(Gem::RDoc)
+
 class TestGemRDoc < Gem::TestCase
   def setup
     super