common.py: handle missing "runtime" job result field

[kaspar030]

"dwq-subjob" might send these.

Traceback (most recent call last):
  File "/opt/murdock-scripts/reporter.py", line 175, in <module>
    main()
  File "/opt/murdock-scripts/reporter.py", line 126, in main
    job = parse_job(job_raw)
  File "/opt/murdock-scripts/common.py", line 26, in parse_job
    result["runtime"] = float(job["result"]["runtime"])
KeyError: 'runtime'

[kaspar030]

Tested on ci-staging here: https://ci-staging.riot-os.org/details/513ec2cf94b849228cb0e322a083c579/tests

[kaspar030]

And here's how the error manifests: https://ci-staging.riot-os.org/details/ff1d811c24ea4a799df12d6df43fa18f