Migrate unit tests hosted in src/wkt/tests/timestamp.rs into wkt/src/timestamp.rs

[mpeddada1]

Context:

Thanks for the context, from reading https://doc.rust-lang.org/book/ch11-03-test-organization.html#:~:text=The%20purpose%20of%20unit%20tests,code%20that%20they're%20testing, it looks like the tests/ file is reserved for integration tests?

Unit tests are small and more focused, testing one module in isolation at a time, and can test private interfaces. Integration tests are entirely external to your library and use your code in the same way any other external code would, using only the public interface and potentially exercising multiple modules per test.

We have a few other tests (like convert_from_time) that might belong in the wkt/src/timestamp.rs. I'll create a follow up to move them.

Originally posted by @mpeddada1 in #559 (comment)

cc/ @coryan