Allow disabling test randomization for certain tests (#1551)

Fixes #1539

Adds an allow_test_randomization option that can be configured for certain tags, in the dart_test.yml file.

Also restructured the Configuration and SuiteConfiguration classes to make future changes less error prone, by making their parameters required. I also added some specialized constructors for the few cases where we really only want to pass a couple options.
17 files changed