test: stabilize tls-cert-leak regression

[mcollina]

Description

Stabilizes test/tls-cert-leak.js by removing the per-request AbortSignal.timeout(2000).

The test already has a test-level timeout, and creating a fresh timeout signal for every failed TLS request retains enough short-lived objects to make this memory regression test flaky under the current runner.

This also recognizes UNABLE_TO_VERIFY_LEAF_SIGNATURE, which is the error code produced locally for the self-signed certificate fixture.

Fixes #5268

Testing

• node --expose-gc test/tls-cert-leak.js
• for i in $(seq 1 10); do node --expose-gc test/tls-cert-leak.js; done
• npx borp --expose-gc -p "test/tls-cert-leak.js"

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.23%. Comparing base (8755693) to head (54b8781).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5306   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files         110      110           
  Lines       36575    36575           
=======================================
+ Hits        34100    34101    +1     
+ Misses       2475     2474    -1     

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