Fix spec in shell_spec.rb

The spec for "#add_key does nothing" would always have passed,
since the expectation was on both the wrong object and message.

spec/lib/gitlab/shell_spec.rb

@@ -69,7 +69,7 @@ describe Gitlab::Shell do
       end
 
       it 'does nothing' do
-        expect(Gitlab::Utils).not_to receive(:system_silent)
+        expect(gitlab_shell).not_to receive(:gitlab_shell_fast_execute)
 
         gitlab_shell.add_key('key-123', 'ssh-rsa foobar trailing garbage')
       end