Add integration tests for xDS flow control

[hanbingleixue]

What type of PR is this?

Enhancement

What this PR does / why we need it?

Add integration tests for xDS flow control

Which issue(s) this PR fixes？

Fixes #1740

Does this PR introduce a user-facing change?

No

Checklist

• Make sure there is a GitHub_issue related with this PR before you start working on it.
• Make sure you have squashed your change to one single commit.
• GitHub Actions works fine in this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../retry/condition/ConnectFailureRetryCondition.java	0.00%	7 Missing ⚠️
...ommon/xds/retry/condition/ResetRetryCondition.java	0.00%	5 Missing ⚠️

Flag	Coverage Δ	Complexity Δ
unittests	27.69% <0.00%> (-0.01%)	156.00 <0.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ	Complexity Δ
...ommon/xds/retry/condition/ResetRetryCondition.java	5.55% <0.00%> (-0.33%)	0.00 <0.00> (ø)
.../retry/condition/ConnectFailureRetryCondition.java	5.55% <0.00%> (-1.59%)	0.00 <0.00> (ø)

... and 1 file with indirect coverage changes