test: move testing submodule to fork

[beckermr]

This PR moves the submodule we use for tests to a fork per discussion with @rmjarvis.

[codspeed-hq]

Merging this PR will improve performance by 60.82%

⚠️ 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

⚡ 6 improved benchmarks
✅ 30 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	test_benchmark_interpimage_flux_frac[run]	2.4 ms	1.9 ms	+30.11%
⚡	Simulation	test_benchmarks_metacal[run]	178.2 ms	132.4 ms	+34.62%
⚡	WallTime	test_benchmark_moffat_conv[run]	290.1 ms	195.9 ms	+48.08%
⚡	WallTime	test_benchmark_moffat_conv_grad[run]	892.5 ms	666.3 ms	+33.94%
⚡	WallTime	test_benchmarks_metacal[run]	18.4 ms	12.9 ms	+42.28%
⚡	WallTime	test_benchmark_spergel_conv[run]	265.2 ms	164.9 ms	+60.82%

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_{Comparing move-testing-submodule (ab8ac61) with main (d2f2d94)}