security: add unit test for cert pool

[djshow832]

What problem does this PR solve?

Issue Number: ref #464

Problem Summary:
It proves that a cert pool can store multiple CAs and each CA works. It's useful for the design of cert-based auth.
E.g. there are 2 certs: ca1 + cert1 + key1 and ca2 + cert2 + key2
If I put both ca1 and ca2 into the cert pool, the cert pool can verify both cert1 + key1 and cert2 + key2.

What is changed and how it works:
Add a unit test.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@b543b47). Click here to learn what that means.

Additional details and impacted files

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  Files           ?       61           
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  Branches        ?        0           
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  Hits            ?     3766           
  Misses          ?     1755           
  Partials        ?      293           

Flag	Coverage Δ
unit	64.77% <ø> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-08 01:26:08.460690135 +0000 UTC m=+839229.988230673: ☑️ agreed by xhebox.