tests: collect all SIGCHLD before cleaning event loop
tests: collect all SIGCHLD before cleaning event loop

On python 3.6.4 apparently it requires two callbacks runs to cleanup
stale SIGCHLD handlers.

QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin