Replace WorkspaceStack unit tests with an irb_load test

The unit tests repeated what the workspace command tests already cover
through real sessions. Test the behavior change instead: a file loaded
into a private module with irb_load now has helper methods like conf.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0175QjMQGeUPByLSqoL6ye5U

Author: claude

test/irb/test_command.rb

@@ -450,6 +450,15 @@ def test_irb_load
         ], out)
     end
 
+    def test_irb_load_in_private_module_has_helper_methods
+      File.write("#{@tmpdir}/a.rb", "conf.class\n")
+      out, err = execute_lines(
+        "irb_load '#{@tmpdir}/a.rb', true\n",
+      )
+      assert_empty err
+      assert_match(/=> IRB::Context\n/, out)
+    end
+
     def test_irb_load_without_argument
       out, err = execute_lines(
         "irb_load\n",