capture and assert warning in test suite

spec/lib/docbookrx/docbookrx_spec.rb

@@ -474,9 +474,11 @@
 | Dam
 |===
     EOS
-    output = Docbookrx.convert input
-
-    expect(output).to include(expected)
+    capture_stdio do |_, stderr|
+      output = Docbookrx.convert input
+      expect(output).to include(expected)
+      expect(stderr.string).to include('5 columns specified in table \'Control parameters\', but only 4 headers')
+    end
   end
 
   it 'should convert nested program listings in listitems correctly' do

spec/support/extensions/helpers.rb

@@ -0,0 +1,15 @@
+require 'stringio'
+
+module RSpec::Support::Helpers
+  def capture_stdio
+    old_stdout, $stdout = $stdout, StringIO.new
+    old_stderr, $stderr = $stderr, StringIO.new
+    yield $stdout, $stderr
+  ensure
+    $stdout, $stderr = old_stdout, old_stderr
+  end
+end
+
+RSpec.configure do |config|
+  config.include RSpec::Support::Helpers
+end