test: skip HTTP/2 churn test on Node.js 26

[mcollina]

Skip the HTTP/2 churn stress test on Node.js 26, where nodejs/node#64850 aborts on an internal onread->IsFunction() assertion during session teardown. Preserve the Linux-only Node.js 24 and 25 skip for nodejs/node#64841 and link both upstream issues.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (fc3450d) to head (7a0164d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5742      +/-   ##
==========================================
- Coverage   93.47%   93.46%   -0.01%     
==========================================
  Files         110      110              
  Lines       38908    38908              
==========================================
- Hits        36368    36366       -2     
- Misses       2540     2542       +2     

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