Fix cucumber step '<channel> should not contain anything'

This fixes two things:

- It requires all commands to have no output on the channel, not just
  one.
- It improves the failure message so that it shows the actual output.

features/03_testing_frameworks/cucumber/steps/command/check_stderr_of_command.feature

@@ -66,3 +66,23 @@ Feature: STDERR of commands which were executed
     """
     When I run `cucumber`
     Then the features should all pass
+
+  Scenario: Failure message when checking that stderr from all processes is empty
+    Given a file named "features/output.feature" with:
+    """
+    Feature: Run command
+      Scenario: Run command
+        When I run `bash -c 'printf "hello\nworld!\n" >&2'`
+        And I run `printf "hola"`
+        And the stderr should not contain anything
+    """
+    When I run `cucumber`
+    Then the features should not pass with:
+    """
+          expected "hello
+    world!" to output string is eq: ""
+          Diff:
+          @@ -1,2 +1,4 @@
+          +hello
+          +world!
+    """

lib/aruba/cucumber/command.rb

@@ -450,14 +450,10 @@
   expect(last_command_stopped).to have_output an_output_string_matching(expected)
 end
 
-Then(/^(?:the )?(output|stderr|stdout) should not contain anything$/) do |channel|
-  matcher = case channel
-            when 'output'; then :have_output
-            when 'stderr'; then :have_output_on_stderr
-            when 'stdout'; then :have_output_on_stdout
-            end
+Then '(the ){channel} should not contain anything' do |channel|
+  combined_output = send(:"all_#{channel}")
 
-  expect(all_commands).to include send(matcher, be_nil.or(be_empty))
+  expect(combined_output).to output_string_eq ''
 end
 
 Then(/^(?:the )?(output|stdout|stderr) should( not)? contain all of these lines:$/) \