Use 'test-setting' to extract platform / build / runtime data out of test tasks

[marco-c]

Equivalent to mozilla/mozci#557.

In https://bugzilla.mozilla.org/show_bug.cgi?id=1632870, we added a new test-setting property to the extra section of test tasks. We should parse this to obtain platform / build / runtime properties of test tasks rather than relying on the label.