intermittent qtest-aarch64/xlnx-canfd-test test failure

[Michael Tokarev]

Hi!

While doing stable-8.1.1 preparation, I've a CI failure of 
ubuntu-20.04-s390x-all here:

https://gitlab.com/qemu-project/qemu/-/jobs/5132720046

▶ ERROR:../tests/qtest/xlnx-canfd-test.c:265:match_rx_tx_data: \
  assertion failed ((buf_rx[size] & DLC_FD_BIT_MASK) == (buf_tx[size] & 
DLC_FD_BIT_MASK)): (2281701376 == 2147483648)

 qemu:qtest+qtest-aarch64 / qtest-aarch64/xlnx-canfd-test  \
           ERROR           0.77s   killed by signal 6 SIGABRT

Re-run of this test went successfully.  However, the assert looks
a bit scary, and the fact that it does not fail in a reliable way
is a bit troubling too.  Is it a flaky test or there's something
else in qemu with concurrent threads?

I don't see this assert in previous test runs.  But ubuntu-20.04-s390x-all
test fails quite often due to other reasons, so it isn't conclusive.

Should something be done with this?

The testing is done for commit f2fc49c302036315db6e8c9f74592decc3be0476,
which is in staging-8.1 branch only temporarily.

Thanks,

/mjt