[swarming] add CAS input/output integration tests
[swarming] add CAS input/output integration tests

Tasks with CAS go though different code path from with Isolate server.
This CL adds integration tests for CAS input/output scenarios.

Bug: 1173322
Change-Id: I46b643153eb0bac817940aa26031490441454a19
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2671440
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go