tests: more qrexec tests, this time for deadlocks
tests: more qrexec tests, this time for deadlocks

Tests for recently discovered deadlocks on write(2).

QubesOS/qubes-issues#1347

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