test-blockjob: intermittent CI failures in msys2-64bit job

[Peter Maydell]

I've noticed that test-blockjob seems to fail intermittently
on the msys2-64bit job:

https://gitlab.com/qemu-project/qemu/-/jobs/3872508803
https://gitlab.com/qemu-project/qemu/-/jobs/3871061024
https://gitlab.com/qemu-project/qemu/-/jobs/3865312440

Sample output:
| 53/89 ERROR:../tests/unit/test-blockjob.c:499:test_complete_in_standby:
assertion failed: (job->status == JOB_STATUS_STANDBY) ERROR
53/89 qemu:unit / test-blockjob ERROR 0.08s exit status 3

I haven't looked at whether these jobs all failed on the
same subtest or whether there is better info lurking in
some log file in the pipeline's artefacts.

thanks
-- PMM