test: no need to set stepk anymore

[beckermr]

Interpolated images now compute stepk just like galsim, so no need to set it in the tests.

closes #222

[codspeed-hq]

Merging this PR will improve performance by ×2.2

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 11 improved benchmarks
✅ 25 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_benchmarks_lanczos_interp[kval-conserve_dc-run]	55.5 µs	31.4 µs	+76.88%
⚡	WallTime	test_benchmark_moffat_init[run]	95.4 µs	43.7 µs	×2.2
⚡	WallTime	test_benchmark_invert_ab_noraise[run]	134.5 µs	95.4 µs	+41.02%
⚡	WallTime	test_benchmark_spergel_kvalue[run]	11.8 ms	8.8 ms	+34.14%
⚡	WallTime	test_benchmark_spergel_init[run]	6.7 µs	4.9 µs	+36.06%
⚡	WallTime	test_benchmark_spergel_calcfluxrad[run]	212.2 µs	139.3 µs	+52.32%
⚡	WallTime	test_benchmark_gaussian_init[run]	6.7 µs	5.1 µs	+32.65%
⚡	WallTime	test_benchmark_interpimage_flux_frac[run]	421.4 µs	250.8 µs	+68.03%
⚡	WallTime	test_benchmarks_lanczos_interp[kval-no_conserve_dc-run]	55.9 µs	30.7 µs	+82.05%
⚡	WallTime	test_benchmarks_lanczos_interp[xval-no_conserve_dc-run]	114.3 µs	65.2 µs	+75.38%
⚡	WallTime	test_benchmarks_lanczos_interp[xval-conserve_dc-run]	131.8 µs	75.9 µs	+73.58%

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_{Comparing remove-force-stepk (5493be9) with main (c8714ed)}