[core] Place unit test alongside with the implementation

[dentiny]

Description

Bazel's general practice for C++ is to place header file, impl file and unit test files under the same folder.
Example for abseil: https://github.com/abseil/abseil-cpp/blob/master/absl/container/BUILD.bazel

Use case

No response

[nishi-t]

Hi, I would like to contribute to Ray. May I work on this issue?

[dentiny]

Hi @nishi-t , I think this issue needs more discussion.

Meanwhile, I have some other good first issues if you're willing to take part in

• [core] Cover cpplint for all C++ folders #50583
• [core] Split giant ray core C++ targets into small ones #50586
• bazel-lint all BUILD files #50875

Let me know if you're interested in any!

[nishi-t]

@dentiny
Thank you for sharing these issues!
I will check them and find one that I can work on.
I will let you know once I decide.

[dentiny]

This issue needs further discussion, close it for now.