Validate tests in submission results

[chaws]

We get 500 error:

File "squad/core/data.py" in parse_test_result
  16.     v = value.lower()

Exception Type: AttributeError at /api/submit/armnn/armnn/1983398/synquacer
Exception Value: 'list' object has no attribute 'lower'

Where POST contains

tests = '{"mlperf_suite/Execution_time_sec": [45773.969]...

We should probably return 400 and include a validation message in the response body