test: migrate stats/vartest to ULP-based assertions

[kgryte]

Resolves a part of #11352.

Description

What is the purpose of this pull request?

This pull request:

• migrates the unit tests for stats/vartest from relative tolerance assertions to ULP difference assertions, as proposed in [RFC]: Migrate math/base/special packages from relative tolerance testing to ULP difference testing (tracking issue) #11352.
• replaces each delta/tol computation and corresponding t.ok( delta <= tol, ... ) assertion with t.strictEqual( isAlmostSameValue( actual, expected, <ULP> ), true, 'returns expected value' );.
• adds a @stdlib/assert/is-almost-same-value require and removes the now unused @stdlib/math/base/special/abs and @stdlib/constants/float64/eps requires.

Only test/test.js is modified; no implementation, documentation, or benchmark files are touched. Pre-existing exact assertions (e.g., out.pValue for the ratio option fixture, the 0.0 lower CI bound, and the +Infinity upper CI bound) are left as-is, as exact comparisons remain correct for those cases.

The ULP bounds below are the measured minimums for each comparison: the tests pass at the stated value and fail at one ULP lower, as verified by computing number/float64/base/ulp-difference for every assertion.

Fixture / quantity	Previous tolerance	ULP bound (measured minimum)
twosided.pValue	250.0 * EPS	253
twosided.statistic	1.0 * EPS	1
twosided.lower (lower CI)	5.0 * EPS	8
twosided.upper (upper CI)	10.0 * EPS	9
diff.statistic	1.0 * EPS	1
diff.lower (lower CI)	6.0 * EPS	8
diff.upper (upper CI)	12.0 * EPS	9
less.pValue	5.0 * EPS	4
less.statistic	1.0 * EPS	1
less.upper (upper CI)	5.0 * EPS	8
greater.pValue	250.0 * EPS	253
greater.statistic	1.0 * EPS	1
greater.lower (lower CI)	1.0 * EPS	1

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.

• make test TESTS_FILTER=".*/stats/vartest/.*" passes (83/83). The suite was run three times at the final ULP values to confirm the results are deterministic.
• make lint-javascript-tests TESTS_FILTER=".*/stats/vartest/.*" is clean.

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 by Claude Code, mirroring the migration idiom established by previously merged conversions in this family (e.g., stats/base/dists/exponential/variance, stats/levene-test). The ULP bounds were measured empirically rather than guessed.

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@stdlib-js/reviewers

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Generated by Claude Code

[stdlib-bot]

Hello! 👋

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/vartest	$\\color{green}388/388$ $\\color{green}+100.00\\%$	$\\color{green}60/60$ $\\color{green}+100.00\\%$	$\\color{green}3/3$ $\\color{green}+100.00\\%$	$\\color{green}388/388$ $\\color{green}+100.00\\%$

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