qa/run_xfstests_qemu.sh: harden against wget failures

qa/run_xfstests_qemu.sh: harden against wget failures

If wget fails (e.g. due to a certificate issue), it still creates
an empty file.  Then this file is marked executable, ./"${SCRIPT}"
immediately returns 0 and run_xfstests_qemu.sh exits successfully
without running a single xfstest.

This started on Sep 30, 2021 with the expiration of Let's Encrypt
root certificate -- all qemu jobs with "test: qa/run_xfstests_qemu.sh"
just booted the VM for a couple of seconds and reported success.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>