Add test package for single and multi thread

[icsl-Jeon]

Hello. My name is Felipe Jeon, and I am glad to open a PR regarding comparisons on SingleThreadedExecutor and MultiThreadedExecutor (reentrant and mutually exclusive). I attached detailed analysis on the link inside README.md.

Especially, I covered

• Round robin of SingleThreadedExecutor
• Thread scheduling of Reentrant callback group
• Mutex and MutuallyExclusive callback group

If you have any comments or guidance, please let me know 😊