test: add unit tests for rolling file

[1996fanrui]

close #34

Changes:

• refactor: move Rotation into one separate module and add test for it
  • The rolling file has too many code, so move Rotation into one separate module
• test: add unit tests for rolling file
  • Introduce the Clock related code to test time rotation

[1996fanrui]

Hey, I added a few tests for rolling file. Please let me know if we need more tests to cover more cases.

Also, it's my first time to use rust. Feel free to let me know if my change doesn't make sense, thanks a lot.

[tisonkun]

Let's keep the name Rotation as we don't introduce other rotation strategy. Also this is a public struct that changing its name would cause downstream software to migrate.

This PR adds tests only, and thus should avoid breaking changes.

[tisonkun]

You can use --all-features, i.e.,

cargo test --all-features -- --nocapture

[tisonkun]

Just default to DefaultClock?

[1996fanrui]

Updated, and I simplified the Clock related code as well.

[tisonkun]

Thanks for your contribution!

Comments above. Rest LGTM.

1. You may try to add cases that rotates by both time and size;
2. You can run cargo +nightly fmt --all to apply all the format rules (some of them are unavailable in the stable channel).

[1996fanrui]

Thanks for your contribution!

Comments above. Rest LGTM.

1. You may try to add cases that rotates by both time and size;

2. You can run `cargo +nightly fmt --all` to apply all the format rules (some of them are unavailable in the stable channel).

Thanks for the review, all comments are addressed. And run cargo +nightly fmt --all before commit.

[tisonkun]

Thank you!