bench: update random value generation

[hrshya]

Resolves none .

Description

What is the purpose of this pull request?

This pull request:

• Refactors random number generation in JS benchmarks for the math/base/special/copysign, math/base/special/cosm1, math/base/special/cospi and math/base/special/cosh packages.
• Replaces randu() with uniform() from @stdlib/random/array/uniform for cleaner and more consistent code.
• Moves the random number generation outside the benchmarking loops.
• Updates the test messages to follow code conventions.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves no related issues.

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.

---

@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/copysign	$207/207$ $+100.00\mathrm{\%}$	$5/5$ $+100.00\mathrm{\%}$	$2/2$ $+100.00\mathrm{\%}$	$207/207$ $+100.00\mathrm{\%}$
math/base/special/copysignf	$203/203$ $+100.00\mathrm{\%}$	$5/5$ $+100.00\mathrm{\%}$	$2/2$ $+100.00\mathrm{\%}$	$203/203$ $+100.00\mathrm{\%}$
math/base/special/cosm1	$242/242$ $+100.00\mathrm{\%}$	$12/12$ $+100.00\mathrm{\%}$	$3/3$ $+100.00\mathrm{\%}$	$242/242$ $+100.00\mathrm{\%}$
math/base/special/cospi	$217/217$ $+100.00\mathrm{\%}$	$20/20$ $+100.00\mathrm{\%}$	$2/2$ $+100.00\mathrm{\%}$	$217/217$ $+100.00\mathrm{\%}$

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

[anandkaranubc]

/stdlib lint-autofix

[anandkaranubc]

Everything looks good! Thanks @hrshya

[kgryte]

LGTM