chore: modify check execution from multi-threaded to single-threaded

[nathanwn]

This pull request modifies the check execution in Macaron from a multi-threaded implementation to a single-threaded one.

This change is for a couple of reasons:

• The multithreaded solution was initially for performance purposes. However, parallelizing check execution may not be the best approach for speeding up Macaron. In addition, Macaron almost never runs with multithreading (2 or more runners) enabled.
• The new single-threaded implementation makes it easier for debugging and profiling since it is better compatible with native Python toolings (e.g. cProfile and pdb).
• The new single-threaded implementation also simplifies the check execution implementation and makes it easier to maintain.

[tromai]

https://github.com/oracle/macaron/pull/400 can be closed after this Pull Request is merged.