prod: enable parallel testing for faster tests and fix codespeed deprecation

[beckermr]

Hopefully the CI is a bit less painful with this.

I also fixed a deprecation coming in codspeed.

closes #166

[codspeed-hq]

Merging this PR will degrade performance by 38.29%

❌ 2 (👁 1) regressed benchmarks
✅ 29 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
👁	test_benchmarks_lanczos_interp[xval-no_conserve_dc-run]	677.3 µs	1,097.4 µs	-38.29%
❌	test_benchmark_spergel_calcfluxrad[run]	1.1 ms	1.5 ms	-28.25%

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_{Comparing beckermr-patch-1 (22351cf) with main (40845dd)}

[beckermr]

Even with two workers GitHub is killing the jobs. That means we'll need split tests if we want this to be faster.