Fix issue in TestRDocServlet, also happens on CRuby

* `Gem::Specification.all = [@SPEC]` is a bad hack and is what breaks it,
  but that's what the test does, even for latest CRuby.

Author: eregon

test/mri/tests/rdoc/test_rdoc_servlet.rb

@@ -1,6 +1,12 @@
 # frozen_string_literal: true
 require_relative 'helper'
 
+# Load those eagerly, otherwise require 'json' inside fails due to `Gem::Specification.all = [@spec]`, also on CRuby
+_load_eagerly = [
+  RDoc::Servlet,
+  RDoc::Generator::JsonIndex
+]
+
 class TestRDocServlet < RDoc::TestCase
 
   def setup