test: add unexpected disconnect guards to remaining http2 tests

[zeexzeex]

Adds the disconnect guard pattern from docs/docs/best-practices/writing-tests.md to the remaining happy-path HTTP/2 test files, so silent disconnect/reconnect cycles fail the tests instead of passing unnoticed. Follow-up to #5533.

Files covered (13 guards): http2-agent.js (1, on an Agent), http2-alpn.js (2), http2-trailers.js (1, converted to tspl since every existing guard relies on tspl's t.fail), http2-dispatcher.js (9, including the forward client in Dispatcher#Connect).

Intentionally skipped: the upgrade/abort/destroy/reject tests in http2-dispatcher.js (disconnect expected or no connection), http2-stream.js (server destroys every stream), http2-max-concurrent-streams-zero.js (drain behavior under test), and 4 unit-test files that mock the h2 internals with no real connection (http2-late-data.js, http2-response-after-completion.js, http2-window-size.js, http2-resume-null-request.js).

Verified: all 23 tests pass, 10 consecutive full runs with no flakes, and a manual spot check confirmed the guard turns a mid-test session teardown into a failure instead of a silent reconnect.

Refs: #251

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (dd85997) to head (f7223e6).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5686      +/-   ##
==========================================
+ Coverage   93.43%   93.44%   +0.01%     
==========================================
  Files         110      110              
  Lines       38733    38776      +43     
==========================================
+ Hits        36190    36236      +46     
+ Misses       2543     2540       -3     

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