make integration tests support checking more than one span of output #86

The current "span_data" check in the test fixtures only supports one span. That's a result of the way I originally wrote it for one span then expanded to multiple and haven't used it much. In working with @edw-eqix on some test PRs we realized this.

I think the JSON would end up looking like:

[
  {
    "expect": {
      "output": "hello world\n",
      "spans": 2,
      "span_data": [
            {
              "trace_id": "edededededededededededededed9000",
              "span_id": "*",
              "is_sampled": "true"
            },
            {
              "trace_id": "*",
              "span_id": "*",
              "is_sampled": "true"
            }
      ]
    }
  }
]

So we have a list of spans to check instead of a single span hash.

A trick here is that technically spans can come into the server out of order. The simplest way forward for now is probably to call sort.Sort() on a CliEventList of spans. This will require a little refactoring and maybe extra work to make sure all spans are captured and passed back correctly.