update BufferPoolWatermarkTest for cisco-8000 hbm

[zhixzhu]

Description of PR

After enable hbm for long-link, the bufferpool for lossless is hbm.
Before packets are evicted to hbm, the bufferpool watermark will always be 0.
Once hit the eviction threshold, the whole voq is evicted to hbm, and the bufferpool watermark will be increased once by a large value.

Send pkts_num_fill_ingr_min packets as initial, which is one packet less than eviction threshold, bufferpool watermark is 0.
Before the while loop, set expected_wm = pkts_num_fill_min * cell_occupancy.
Then in while loop, the expected_wm will be added up to the correct value.

Summary:
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 201911
• 202012
• 202205

Approach

What is the motivation for this PR?

How did you do it?

How did you verify/test it?

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[wangxin]

@zhixzhu Is this a backport of master branch PR to 202205 branch? Can you mention the master branch PR ID here in PR description?

[zhixzhu]

@zhixzhu Is this a backport of master branch PR to 202205 branch? Can you mention the master branch PR ID here in PR description?

No, I didn't create master branch PR for this.

[wangxin]

@zhixzhu Can you close this one and create PR to master branch instead? After the PR to master branch is merged, then we can backport it to 202205 branch. The master branch is supposed to include everything. If some fixes are only made to release branch, in the future we have to fix it again in master branch and later release branches. There would be duplicated work.