test: make http2-alpn control requests explicit

[trivikr]

This relates to...

Fixes: #5212

Rationale

The HTTP/1.1 control requests in test/http2-alpn.js relied on Node's default https ALPN behavior and asserted alpnProtocol === false.

That ALPN detail is incidental to the behavior this test is meant to verify. The important distinction is that Undici negotiates h2 when allowH2 is enabled, while the same allowHTTP1: true server can still handle an HTTP/1.1 request.

Make the control request explicitly HTTP/1.1 so the test documents that intent directly and avoids depending on the default ALPN behavior of https.

Changes

• Make HTTP/1.1 control requests in test/http2-alpn.js pass ALPNProtocols: ['http/1.1'].
• Update the expected ALPN assertions to http/1.1.
• End the POST control request with its body instead of only writing the body.

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.27%. Comparing base (01122da) to head (1437e4b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5252      +/-   ##
==========================================
- Coverage   93.27%   93.27%   -0.01%     
==========================================
  Files         110      110              
  Lines       36366    36366              
==========================================
- Hits        33922    33921       -1     
- Misses       2444     2445       +1     

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