[release/7.0] Fix missing using in Http3TimeoutTests

[github-actions]

Backport of #43311 to release/7.0

/cc @dougbu @adityamandaleeka

{PR title}

Summary of the changes (Less than 80 chars)

Description

{Detail}

Fixes #{bug number} (in this specific format)

Customer Impact

{Justification}

Regression?

• Yes
• No

[If yes, specify the version the behavior has regressed from]

Risk

• High
• Medium
• Low

[Justify the selection above]

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

---

When servicing release/2.1

• Make necessary changes in eng/PatchConfig.props

[halter73]

@dougbu @wtgodbe @adityamandaleeka @Tratcher @captainsafia Do I have to do a second backport for #43302, or are we going to merge release/7.0-rc1 into release/7.0 and then merge that into main like we were talking about? Option 3 I think?

[wtgodbe]

Does this need to be in rc1? If so this PR should be against that branch instead of this one. rc1 will auto-merge to 7.0 & 7.0 will auto-merge into main (option 3). If #43302 needs to be in rc1, you should manually port it there as well (or port it to this branch if it's for rc2).

[halter73]

The rc1 PR for this change (#43312) has already been merged.

[wtgodbe]

The rc1 PR for this change (#43312) has already been merged.

I see. In that case this PR isn't strictly necessary