As part of https://jira.mongodb.org/browse/TIG-2412, we determined that the best threshold to use is 0. 

As a server engineer, I should know that the threshold used to determine which tasks to run for selected-tests is 0, since that is the threshold that most accurately captures which tasks should be run based on my code changes.

AC:

• Threshold is set to 0 in selected_tests_gen here: https://github.com/mongodb/mongo/blob/36ae8a4824a88bd49ab5fa62740419c10c6bc39d/buildscripts/selected_tests.py#L56.