Heartbeat: set duration to zero for syntax errors

[vigneshshanmugam]

• When there are syntax errors in the monitor script, the journey/start never fires as the Synthetics runner cannot register the journeys and node runs to completion with exit status 1.
• In these cases, determining the duration between start/end event would be tricky and since the journeys never run in the first place, Its good to set the duration to 0 instead of setting false positive values.
• Added extensive test cases for summary event creation for various scenarios

heartbeat.run_once: true
# Configure monitors inline
heartbeat.monitors:
  - type: browser
    id: my-monitor
    name: My Monitor
    source:
      inline:
        script: step("load homepage", async () => {);
    schedule: "@every 1m"

output.console: ~
``

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• Start Time: 2022-02-11T19:20:39.842+0000

• Duration: 48 min 17 sec

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Passed	125
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Total	125

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[andrewvc]

LGTM (also verified the behavior works well on kibana)

[paulb-elastic]

Discussed in Tech Sync to consider having this as null rather than 0 (with a view on if that's supported in the UI)
/cc @vigneshshanmugam

[andrewvc]

LGTM