fix: make sure ftl serve --background waits for controller to start

[wesbillman]

Example:

ftl serve --background && ftl status
info: FTL running in background with pid: 26990

error: Unary RPC failed: unavailable: dial tcp [::1]:8892: connect: connection refused: /xyz.block.ftl.v1.ControllerService/Status
{
  "controllers": [
    {
      "key": "C01HQBDJSK0C249VV2QBN3B856J",
      "endpoint": "http://localhost:8892"
    }
  ],
  "runners": [
...

We'll probably want a way in the future to not print the error in the status request, but it shouldn't stop the Integration/CI flow from working here.

[wesbillman]

Fixes #962