This test uses three tricks to test /var/tmp space monitoring:
1. Creates a big uncompressed backup (2GB file instead of few bytes)
2. Mount small tmpfs over /var/tmp (650MB - minimal space that should
not deadlock the restore)
3. Artificially slow down data processing by adding sleep()
QubesOS/qubes-issues#4791
Based on "backup compatibility" tests, which manually assemble the
backup. This is because we don't have access to actual backup creation
code here.
QubesOS/qubes-issues#1214
