add ut of workload_stream.go

[LiZhenCheng9527]

What type of PR is this? /kind enhancement

What this PR does / why we need it: add ut of workload_stream.go Which issue(s) this PR fixes: a part of #104

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Flag	Coverage Δ
unittests	30.79% <100.00%> (?)

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Files	Coverage Δ
pkg/controller/xdstest/fake_xdsserver.go	77.50% <100.00%> (ø)

[nlgwcy]

/lgtm /approve

[kmesh-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nlgwcy

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