Draft: test demonstrating orphaned process are not killed with their parent

PR: https://github.com/go-gitea/gitea/pull/20264

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This test fails and demonstrates that when Gitea kills one of its children (for instance when mirroring a repository timesout), the grand children are not killed and become orphaned that linger and will eventually become zombies.

This is explained in detail in these blog posts:

• https://hostea.org/blog/zombies/
• https://hostea.org/blog/zombies-part-2/

I'd be happy to work on implementing a bug fix for Gitea.

...
[unit-test:115]         	            	  16511       1   16494 /usr/libexec/git-core/git remote-https origin https://4.4.4.4
[unit-test:116]         	            	  16513   16511   16494 /usr/libexec/git-core/git-remote-https origin https://4.4.4.4
...
[unit-test:120]         	            	  17165   16486       1 ps -x -o pid,ppid,pgid,args
[unit-test:121]         	            	Contains git-remote-https origin https://4.4.4.4
[unit-test:122]         	Test:       	TestManagerKillGrandChildren
[unit-test:123] FAIL
[unit-test:124] coverage: 35.5% of statements
[unit-test:125] FAIL	code.gitea.io/gitea/modules/process	6.518s