[swarming] simplify task_schedler_test.py
[swarming] simplify task_schedler_test.py

This is test refactoring, and doesn't change functionality.

There are many verbose code to call task_scheduler.bot_update_task() since it requires many arguments without defaults.
This CL introduces a helper _bot_update_task() which assigns reasonable defaults in order to simplify tests.

Change-Id: Iff81057b0944a96bfa7c16b94d37a63ec521a3d6
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2094870
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py