internal: add extern crate std to a bunch of test modules

[Person-93]

It's OK to use std for testing when building for no_std.

[codspeed]

Merging this PR will degrade performance by 10.59%

❌ 2 regressed benchmarks
✅ 139 untouched benchmarks
⏩ 6 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	extract_biguint_small	1.4 µs	1.5 µs	-10.78%
❌	extract_biguint_zero	1.4 µs	1.6 µs	-10.41%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing Person-93:no_std-std_in_tests (9c92b95) with main (1f07d9c)}

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩