test: complete the h2 settings round-trip before asserting single-session multiplexing

[ostapondo]

The fetch POST bodies dispatch concurrently (#5494) test flakes on the
macOS runners: sessions.size comes out 2 instead of 1, e.g.
https://github.com/nodejs/undici/actions/runs/30984506686

Over h2 the client counts as busy until the server's SETTINGS frame lands.
With h2c nothing covers that window, so on a slow runner the second fetch
goes out while it is still open and the Agent opens a second connection.

I checked with a TCP proxy that delays server to client by 80ms: no warmup
gives sessions=2 every run, a warmup gives sessions=1 every run.

So warm the session up first, like the SSE test above it does. The #5494
check still works, a client that serialises hangs on secondArrived until
the timeout fires.

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[ostapondo]

Reopened - closing this was my mistake, the flake is still there. Rebased on main so CodeQL goes green.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (6aef2fb) to head (fe72eef).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5656      +/-   ##
==========================================
+ Coverage   93.33%   93.44%   +0.10%     
==========================================
  Files         110      110              
  Lines       38723    38723              
==========================================
+ Hits        36143    36184      +41     
+ Misses       2580     2539      -41     

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