add unit test and cmdset for asyncos

- The prompt specified in issue #3327 works
- Creating a cmdset in order to get a YAML simulation file

examples/device-simulation/cmdsets/asyncos

@@ -0,0 +1,3 @@
+version
+showconfig
+exit

spec/model/asyncos.rb

@@ -0,0 +1,17 @@
+require_relative 'model_helper'
+
+describe 'model/AsyncOS' do
+  before(:each) do
+    init_model_helper
+    @node = Oxidized::Node.new(name:  'example.com',
+                               input: 'ssh',
+                               model: 'asyncos')
+  end
+
+  it 'matches different prompts' do
+    _('(mail.example.com)> ').must_match AsyncOS.prompt
+    _('mail.example.com> ').must_match AsyncOS.prompt
+    # Devices running in cluster mode (see issue #3327)
+    _('(Machine hostname) ').must_match AsyncOS.prompt
+  end
+end