test: migrate stats/base/dists/f/pdf to ULP-based assertions

[aryan7071]

Resolves #11352

Description

What is the purpose of this pull request?

This pull request:

• migrates the test suite for stats/base/dists/f/pdf from relative tolerance (abs/EPS-based) assertions to ULP difference assertions using @stdlib/assert/is-almost-same-value.
• updates test/test.factory.js, test/test.native.js, and test/test.pdf.js. test/test.js contains no fixture-based numeric assertions and required no changes.
• removes the now-unused abs and EPS imports and the corresponding delta/tol variable declarations, collapsing each if/else exact-match-or-tolerance branch into a single isAlmostSameValue assertion.

Each of the six fixture blocks (bothLarge, largeD1, largeD2 each tested against both Julia and Boost C++ reference values) required its own independently-determined minimum ULP bound; a single shared value was not sufficient across all six. Measured minimum ULP bounds:

Fixture	Reference	ULP
both_large	Julia	4003
large_d1	Julia	95
large_d2	Julia	103
both_large	Boost C++	148
large_d1	Boost C++	105
large_d2	Boost C++	1285

Each bound is the exact maximum observed ULP difference over its fixture (1000+ cases each), so it is also the minimum bound that passes.

Two of these bounds are notably larger than the rest: both_large/Julia (4003) and large_d2/Boost C++ (1285). Both outlier cases occur at inputs where d1 is very close to 2.0 (d1=2.0068... and d1=2.2371... respectively) — a value this distribution already treats as a special boundary elsewhere in the test suite (the pdf has a removable discontinuity in its limiting behavior at d1=2, per the existing d1<2.0/d1=2.0/d1>2.0 boundary tests). Inspecting these two cases directly shows actual and expected values still agree closely in absolute/relative terms; the outsized ULP counts reflect the fine representable spacing of floating-point values in this region rather than a correctness issue.

The pre-existing "baseline case" test in test.native.js (a single hardcoded input/output check, unrelated to the six fixture-driven blocks above) already used isAlmostSameValue with ULP=2 prior to this PR and was not modified.

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

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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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• No

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• Test/benchmark generation
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