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 not alter performance

✅ 143 untouched benchmarks
⏩ 6 skipped benchmarks¹

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_{Comparing Person-93:no_std-std_in_tests (4efc168) with main (3260c48)}

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. ↩