Refactor Testsuite check to be inside lind_kill_from_id

[rennergade]

We have to check if were running the test suite so we don't throw common signals such as SIGCHLD on cage exit etc. Right now we do that outside of lind_kill_from_id but it would be better practice to do it inside the function. @qianxichen233 also pointed out there are places were missing this such as generating EPIPE in write etc.