tests: Fix race condition in download fixture

Change-Id: Idace0e9e71a484080fc581e232ce217b449085c1
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17453
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
1 file changed