Add stat reporting interval to JSON .start.test_start

[TheRealDJ]

• Version of iperf3: master

• Issues fixed (if any): Added interval to JSON output start.test_start as a floating point number

• Brief description of code changes (suitable for use as a commit message): Added interval: %f to JSON test_start printf with value test->stats_interval

Example output (master):

# iperf3 --client XXX --omit 0 --time 1 --json | jq .start.test_start
{
  "protocol": "TCP",
  "num_streams": 1,
  "blksize": 131072,
  "omit": 0,
  "duration": 1,
  "bytes": 0,
  "blocks": 0,
  "reverse": 0,
  "tos": 0,
  "target_bitrate": 0,
  "bidir": 0,
  "fqrate": 0
}

Example output (this PR):

# iperf3 --client XXX --omit 0 --time 1 --json | jq .start.test_start
{
  "protocol": "TCP",
  "num_streams": 1,
  "blksize": 131072,
  "omit": 0,
  "duration": 1,
  "bytes": 0,
  "blocks": 0,
  "reverse": 0,
  "tos": 0,
  "target_bitrate": 0,
  "bidir": 0,
  "fqrate": 0,
  "interval": 1
}

[TheRealDJ]

@bmah888 Not sure if you saw this one, but it's a small change just to JSON test_start output. If you need me to re-do my branch due to the Merge branch 'esnet:master' into json_add_stat_interval commit, just let me know. My checks were failing until I merged your fqrate changes.

Thanks.
DJ

[bmah888]

Thanks for the PR! Merging shortly.