Mocks should be synchronized

[Thiht]

With this mock for example:

request:
  method: GET
  path: /foo
context:
  times: 3
response:
  status: 200

If a client runs 10 calls in parallel to /foo, there's a risk that the mock server replies more than 3 times

[gwleclerc]

I used a mutex on mockServer struct so this could not happen anymore