tests: fix FD leak in qrexec test
tests: fix FD leak in qrexec test

Terminate dangling process to avoid FD leak (detected by test cleanup
code).

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