chore: use testcontext for diagnostics-channel tests

[Uzlopak]

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[Copilot]

Pull Request Overview

This PR refactors diagnostics channel test files to use the Node.js test context API instead of the external tspl library. The change simplifies test assertions by replacing tspl(t, { plan: N }) with t.plan(N) and assert.* calls with t.assert.* equivalents.

Key changes include:

• Removing the tspl dependency from all test files
• Converting test plan initialization and assertion methods to use the native Node.js test context API
• Maintaining identical test functionality while using the built-in testing infrastructure

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/node-test/diagnostics-channel/post.js	Replaces tspl assertions with t.assert methods for POST request diagnostics testing
test/node-test/diagnostics-channel/post-stream.js	Updates stream-based POST diagnostics tests to use native test context assertions
test/node-test/diagnostics-channel/get.js	Converts GET request diagnostics tests from tspl to t.assert methods
test/node-test/diagnostics-channel/get-h2.js	Migrates HTTP/2 GET diagnostics tests to use native test context API
test/node-test/diagnostics-channel/error.js	Updates error handling diagnostics tests to use t.assert instead of tspl
test/node-test/diagnostics-channel/connect-error.js	Converts connection error diagnostics tests to native test context assertions

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