Move unit tests behind the public API boundary

[ANONYMOUSZED-beep]

Summary

• move the main test suite from src/tests.rs to tests/smallvec.rs
• update imports to consume smallvec through its public API
• preserve feature-gated coverage for std, serde, and bytes

This makes API export regressions visible to the test suite instead of compiling the tests as an internal crate module.

Closes #440

Validation

• cargo +1.83.0-x86_64-pc-windows-gnu test (67 integration tests and 6 doctests passed)
• cargo +stable-x86_64-pc-windows-gnu test (67 integration tests and 6 doctests passed)
• cargo +nightly-x86_64-pc-windows-gnu fmt --all --check
• cargo +nightly-x86_64-pc-windows-gnu test --all-features (78 integration tests and 6 doctests passed)
• cargo +nightly-x86_64-pc-windows-gnu bench --all-features --no-run
• git diff --check

[jdm]

@ANONYMOUSZED-beep Did you use AI tooling to create this pull request?

[alejandro-vaz]

@jdm I can't tell to be honest, it's 20 LOC, but the PR message is very generic

I think we should just let it pass unless it's obvious, otherwise we're going to go crazy

please merge #462 and this should be mostly solved

[alejandro-vaz]

I'm withholding this PR for a while, it will introduce merge conflicts with #387 #458 #461

[ANONYMOUSZED-beep]

Yes. I used AI-assisted development tooling to help prepare this pull request, and I reviewed the resulting changes before submitting it. I’m happy to provide any additional detail needed under the project’s contribution policy.

[jdm]

Sorry, we have a policy that does not allow that: https://book.servo.org/contributing/getting-started.html#ai-contributions

[jdm]

but the PR message is very generic

That's what I was reacting to.