gh-109817: Add --single-process-per-case option to libregrtest

[Aniketsy]

fixes#109817

• Issue: libregrtest: add option to run every test case in separate process #109817

• still working on this , so converting into draft @serhiy-storchaka please let me know if this looks in right direction or any pointers you want to share. Thanks!

[Aniketsy]

i'll fix these mypy and add tests , in the meantime removing from draft, to get some review and suggestions.

[serhiy-storchaka]

I'm a little busy now, but I'll review it after finishing with other unittest/regrtest tasks. Thank you for your efforts.

[Aniketsy]

I'm a little busy now, but I'll review it after finishing with other unittest/regrtest tasks.

no hurry, i'm improving this till then, so hopefully it creates less pain for you to review 😊 .

I've added tests, with some improvement (locally everything works fine for me) . i'll once again go through this and check for improvement areas before you come here .