tests/avocado/riscv_opensbi.py does not work reliable

[Thomas Huth]

 Hi!

While running a lot of tests (i.e. with a very loaded machine), I noticed 
that tests/avocado/riscv_opensbi.py is very flaky when the host machine is 
slow. I can easily reproduce the problem when running a big compilation job 
on all CPUs in the background and then run the riscv_opensbi.py avocado 
test. One of test_riscv32_spike, test_riscv64_spike, test_riscv32_sifive_u 
or test_riscv64_sifive_u is failing most of the time (but not the virt 
machine tests).

Looking at the logs, it seems like the output sometimes stops somewhere at a 
random place before the boot process reaches the spot that the test is 
looking for. Looking at riscv_htif.c, there does not seem to be any flow 
control implemented here, so I guess at least the spike test is currently 
doomed to fail occasionally. Is there anything that can be done about this 
(e.g. is flow control somehow possible here or does the interface not allow 
this?)? Otherwise, I think it might be best to mark the spike and sifive_u 
tests with QEMU_TEST_FLAKY_TESTS here to make it clear that these tests are 
not reliable by default...?

 Thomas