test: migrate math/base/special/avercos to ULP-based testing

[Planeshifter]

Resolves a part of #11352.

Description

What is the purpose of this pull request?

This pull request:

• migrates test/test.js and test/test.native.js of math/base/special/avercos from relative-tolerance (EPS-based) assertions to ULP-difference assertions via @stdlib/assert/is-almost-same-value.
• uses the minimum required ULP values, verified by direct ULP-difference computation over the test fixtures: data.json requires a maximum of 21 ULPs (a value of 20 fails), and small_negative.json matches exactly (0 ULPs), so the latter test block uses plain t.strictEqual( y, expected[ i ], 'returns expected value' ) assertions.
• applies the same ULP values to the native test file, as the native add-on results are bit-identical to the JavaScript implementation on the tested fixtures (max ULP differences of 21 and 0, respectively), so no compiler-divergence note was needed despite the previous native tolerance (350.0 * EPS) being larger than the JavaScript tolerance (15.0 * EPS).
• retains the EPS require in both files, as it is still used by the NaN-domain tests, and removes the now-unused abs require and delta/tol variables.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: Migrate math/base/special packages from relative tolerance testing to ULP difference testing (tracking issue) #11352

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

Both the JavaScript and native test suites were run locally and pass (24009 assertions each). The minimum ULP values were independently recomputed via @stdlib/number/float64/base/ulp-difference over all fixture cases and confirmed minimal.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

If you answered "yes" above, how did you use AI assistance?

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

This PR was written primarily by Claude Code, including the test migration and the minimum-ULP discovery; the changes were verified by an independent automated review pass and by running the full JavaScript and native test suites locally.

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Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/avercos	$\\color{green}164/164$ $\\color{green}+100.00\\%$	$\\color{green}5/5$ $\\color{green}+100.00\\%$	$\\color{green}2/2$ $\\color{green}+100.00\\%$	$\\color{green}164/164$ $\\color{green}+100.00\\%$

The above coverage report was generated for the changes in this PR.