[cv][test] add helper script to create large CL stacks.
[cv][test] add helper script to create large CL stacks.

Proof:
240-CL long stack https://chrome-internal-review.googlesource.com/c/playground/cq/+/3931400/1
created with
$ ./giant_stack.py -n 240 --cq +2 -w ~/s/tmp/large-cl-stack --description "128-long"
(yes, i made a typo in --description :) ).

R=qyearsley, yiwzhang

Bug: 1217771
Change-Id: I4741d17c3379d3dd441dd245a38e8635bf8e9897
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2983000
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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