Migrate tests/test_gpu_stimp.py to npt.assert_allclose

[viknesh-ai]

Related to #1175

tests/test_gpu_stimp.py (4 sites). Flipped to (cmp, ref) order, atol=1.5e-07. Both sides are clean float64, no casting needed.

Only runs under CUDA, verified locally with NUMBA_ENABLE_CUDASIM=1: 3/3 passing.

To Do List

• assert_allclose, cmp first
• atol=1.5e-07
• cmp not comp
• ref_/cmp_ naming

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[gitnotebooks]

Review these changes at https://app.gitnotebooks.com/stumpy-dev/stumpy/pull/1196

[viknesh-ai]

@seanlaw next one — same conventions as before.