Display total processes used at the end

[Chemaclass]

Status quo

Right now, when running tests in parallel, each test file will be executed in a different child process.

Expected

When running tests in parallel, display the total parallel processes used.

Tech hint

Possible solutions
A: create a global variable and for each runner::call_test_functions & increment it by 1, and later use it
B: use the total files used in runner::load_test_files [this is simpler than A]
C: figure out how to get the possible max child processes created by the system and save it in a variable

While I would consider C is the most correct one, I think B is the simplest and better approach to do first.

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Idea inspired by Pest, eg pest --parallel --processes=10