models/TestResult: Add a metadata JSON field

This makes it easy to store additional information that is meant for
computers, not humans. For example, a CI system may store data such as
the internal run ID associated to the test result.

Signed-off-by: Martin Peres <martin.peres@mupuf.org>
1 job for rest_v5 in 1 minute and 5 seconds (queued for 9 seconds)
latest
Status Job ID Name Coverage
  Test
passed test #12200523

00:01:05