document the values in $ages in FieldTimeIntervalTest

Problem/Motivation

The $ages array should have comments to explain what the 3 different value are.

The first is seconds, the other two are:

> [, $formatted_value, $granularity] = $this->ages[$delta];

Steps to reproduce

Proposed resolution

Merge request link

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet