test: add unexpected disconnect guards to two more h2 tests

[zeexzeex]

Adds the disconnect guard pattern from docs/docs/best-practices/writing-tests.md to two more HTTP/2 test files, so a silent disconnect/reconnect cycle fails the test instead of passing unnoticed. Follow-up to #5686.

Files covered (3 guards): http2-informational.js (1), http2-timeout.js (2).

Of the 21 http2-* files still without a guard, these two are the only ones where an unexpected disconnect is a fault. The rest fall into four groups: the disconnect is deliberate (http2-goaway*.js, http2-abort.js, http2-refused-stream.js, http2-stream.js, http2-request-never-settles.js, http2-max-concurrent-streams-zero.js), the h2 internals are mocked with no real connection (http2-late-data.js, http2-window-size.js, http2-response-after-completion.js, http2-ping-errors.js, http2-invalid-session.js), no server is started at all (http2-resume-null-request.js, http2-stream-internal-error.js, http2-destroy-after-failed-stream.js, http2-goaway-retry-body.js), or teardown itself is under test (http2-stream-cancel.js, http2-instantiation.js).

http2-invalid-connection-headers.js looked like a candidate but is not one: the session does close there, and the test already asserts connectCount === 2 for the reconnect that follows.

http2-body.js and the connect-* group are left for the follow-up already announced in #251.

Verified: both files pass with the guards in place, and lint is clean. Note that neither guard turns a currently failing case green; they are there to catch a regression, which is what the issue asks for.

Refs: #251

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.53%. Comparing base (bfea020) to head (61ce494).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5825   +/-   ##
=======================================
  Coverage   93.53%   93.53%           
=======================================
  Files         110      110           
  Lines       39773    39773           
=======================================
  Hits        37200    37200           
  Misses       2573     2573           

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