This PR improves the gen_defaults tests to not be tied to a particular order of execution.
Since there is a global class that holds the state of the used/not used tokens, we need to clear this logger before each test.

Fixes https://github.com/flutter/flutter/issues/142716

cc @zanderso @QuncCccccc 

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*