Try to make async ^C spec less flaky

Was intermittently failing, sleep for a little longer to increase
chances of false negatives.

spec/async_spec.rb

@@ -34,9 +34,9 @@ context 'with asynchronous suggestions enabled' do
 
     it 'terminates the prompt and begins a new one' do
       session.send_keys('e')
-      sleep 0.1
+      sleep 0.5
       session.send_keys('C-c')
-      sleep 0.1
+      sleep 0.5
       session.send_keys('echo')
 
       wait_for { session.content }.to eq("e\necho")