test: migrate math/base/special/gammaincinv to ULP-based assertions

[aryan7071]

Resolves a part of #11352

Description

What is the purpose of this pull request?

This pull request:

• migrates the test suite for math/base/special/gammaincinv from relative tolerance (abs-based) assertions to ULP difference assertions using @stdlib/assert/is-almost-same-value.
• updates test/test.js only. test/test.native.js was already migrated to ULP-based testing prior to this PR and is unchanged.
• removes the now-unused abs import.
  The ULP bound was tightened empirically: starting from the maximum observed ULP difference across each fixture set and confirming that one less fails at least one assertion.

Measured minimum ULP bounds (test/test.js):

Fixture	ULP
general (arg1/arg2/expected)	3.0
small p (p < 1e-4)	375.0
large p (p > 0.9)	45.0

Each bound is the exact minimum that passes: lowering any of the three by 1 causes at least one assertion to fail. The pre-existing "upper incomplete gamma" block (already using isAlmostSameValue with 40.0) was not touched.

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

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No.

Checklist

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

• Read, understood, and followed the contributing guidelines.

AI Assistance

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Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/gammaincinv	$\\color{red}1647/1872$ $\\color{green}+0.00\\%$	$\\color{red}129/160$ $\\color{green}+0.00\\%$	$\\color{red}16/24$ $\\color{green}+0.00\\%$	$\\color{red}1647/1872$ $\\color{green}+0.00\\%$

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