Migrate tests/test_gpu_aamp_stimp.py to npt.assert_allclose

[viknesh-ai]

Related to #1175

Covers tests/test_gpu_aamp_stimp.py (2 call sites, small one). Already used ref_/cmp_ naming, just needed the order flip and assert_allclose conversion.

Both PAN/pan values are plain float64 on both sides (confirmed under CUDA sim), no casting needed.

Only runs under CUDA — locally it's a module-level skip, verified with NUMBA_ENABLE_CUDASIM=1: 2/2 passing.

To Do List

• assert_almost_equal -> assert_allclose
• no rtol=0
• actual, desired order
• atol=1.5e-07 - no decimal=/config references here
• cmp not comp - already used cmp_
• ref_/cmp_ naming - already correct, no casting needed

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[gitnotebooks]

Review these changes at https://app.gitnotebooks.com/stumpy-dev/stumpy/pull/1192

[viknesh-ai]

@seanlaw next file in the split — same conventions as the previous PRs. Ready for review whenever you have time.