Follow a more intuitive time duration format for the `time_limit` field in `RegressionTest`

[vkarak]

The current one is just convenient from the implementation point of view (kind of), but a bit clumsy and unintuitive when you have to set the field. I suggest using a standard ISO 8601 or RFC 3339 conformant format.

[vkarak]

ISO and RFC are very generic in describing durations and their format is not very readable. For example, P1DT1H20M30S is used to express a duration of 1 day, 1 hour, 20 minutes and 30 seconds. It also allows you to define durations in weeks and years. A simpler format would much easier to read, such as, 1d20m30s (and supporting only days).