[swarming] fix tools.clear_cache_all
[swarming] fix tools.clear_cache_all

clear_cache_all() doesn't invoke clear_cache() as expected, and caused flaky test failures.
https://logs.chromium.org/logs/infra/buildbucket/cr-buildbucket.appspot.com/8888134910469147232/+/steps/swarming_bot/0/steps/run_tests_python3/0/stdout

Change-Id: I30e869462a9ed0c202946b727f3bb97af8996131
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2061828
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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