tests: adjust for qrexec-client-vm exit code
tests: adjust for qrexec-client-vm exit code

It now consistently returns remote exit code, not a local one.
Check 'cat' exit code instead.

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