Update test syntax to remove stubs (#1138)

Use of stubs are deprecated, switch to the newer expect/allow syntax.

spec/support/libvirt_context.rb

@@ -25,6 +25,6 @@ shared_context 'libvirt' do
     # return some information for domain when needed
     allow(domain).to receive(:mac).and_return('9C:D5:53:F1:5A:E7')
 
-    machine.stub(id: id)
+    allow(machine).to receive(:id).and_return(id)
   end
 end

spec/support/sharedcontext.rb

@@ -28,7 +28,7 @@ shared_context 'unit' do
   let(:plugin)           { register_plugin }
 
   before (:each) do
-    machine.stub(guest: guest)
-    machine.stub(communicator: communicator)
+    allow(machine).to receive(:guest).and_return(guest)
+    allow(machine).to receive(:communicator).and_return(communicator)
   end
 end