tests: fix checking exit code
tests: fix checking exit code

Since run_for_stdio raise an exception for non-zero exit code, it isn't
ignored anymore. So, check if qrexec-client-vm return expected value,
instead of keep ignoring it.

QubesOS/qubes-issues#2861

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