chore: use testcontext for test:unit files, batch 1

[Uzlopak]

I have still about 88 file in my local repo, which need to be upstreamed, but these files should already pass

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• Tested
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[Copilot]

Pull Request Overview

This PR converts test files from using the external tspl testing library to Node.js's native test context (t) parameters. The primary purpose is to modernize the testing approach by removing the dependency on @matteo.collina/tspl and leveraging Node.js built-in testing capabilities.

Key changes include:

• Replacing tspl(t, { plan: n }) with t.plan(n)
• Converting assertion calls from strictEqual, throws, etc. to t.assert.strictEqual, t.assert.throws, etc.
• Updating test function signatures to use test context parameters appropriately

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/util.js	Converted assertions to use test context, removed tspl import
test/trailers.js	Updated test functions to use (t, done) pattern, converted assertions
test/tls.js	Updated commented-out assertions to use test context format
test/tls-session-reuse.js	Converted from async tspl pattern to sync test context
test/tls-cert-leak.js	Removed assert and tspl imports, updated assertions
test/timers.js	Converted all timer tests to use test context assertions
test/snapshot-testing.js	Extensive conversion of assertions to test context format
test/snapshot-redirect-interceptor.js	Updated assertions to use test context
test/snapshot-recorder.js	Converted all recorder tests to use test context
test/redirect-request.js	Updated redirect tests with test context and callback patterns
test/redirect-pipeline.js	Converted pipeline tests to use test context
test/readable.js	Updated readable stream tests with test context
test/proxy.js	Converted proxy tests to use test context assertions
test/proxy-agent.js	Extensive conversion of proxy agent tests
test/invalid-headers.js	Updated header validation tests
test/install.js	Converted installation tests to use test context
test/fixed-queue.js	Updated queue tests with test context
test/examples.js	Converted example tests to use test context
test/errors.js	Updated error handling tests
test/dispatcher.js	Converted dispatcher tests to use test context

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