[swarming] Allow customizing test task ids.
[swarming] Allow customizing test task ids.

Allow customizing the task ids used in recipe tests. Previously all
trigger() calls started with zero, but that causes conflicts when
trigger() is called multiple times in the test.

Change-Id: Ic9af8fd562f9f9f2da541fbf881b55676afc9899
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Recipe-Nontrivial-Roll: chromiumos
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1773434
Reviewed-by: Joshua Seaton <5db77f46805dafe411bd82a965fc09402751eb6e@google.com>
Commit-Queue: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>

luci/recipes-py,luci/recipes-py