Spread tests fail on openstack: "remote directory /root/proj ... is not empty" #1200

Bug Description

The test-cmd spread test fails on the openstack backend. testcraft test starts an inner spread run using its generated craft adhoc backend, which fails while sending project content: /root/proj already exists and is non-empty on the freshly allocated system, so all three allocation retries are discarded and the run aborts. The failure happens before any test or backend prepare runs, so the directory must be created by ./spread/.extension allocate ci or shipped in the openstack image.

The test passes on multipass (same commit, same seed), both as a single task and as part of the full multipass:ubuntu-24.04-64 suite, so this appears openstack-specific.

Likely we'll need to look at what's actually in /root/proj using spread -debug to determine the provenance of the stuff in that directory. I suspect it might be the outer project.

To Reproduce

Run the test-cmd spread test on the openstack backend, e.g. the CI job for openstack:ubuntu-24.04-64:tests/spread/testcraft/test-cmd.

Relevant log output

2026-09-28 17:06:21.840 :: Allocating craft:ubuntu-24.04...
2026-09-28 17:06:22.374 :: Waiting for craft:ubuntu-24.04 to make SSH available at localhost...
2026-09-28 17:06:22.375 :: Allocated craft:ubuntu-24.04.
2026-09-28 17:06:23.278 :: Connecting to craft:ubuntu-24.04...
2026-09-28 17:06:23.363 :: Connected to craft:ubuntu-24.04 at localhost.
2026-09-28 17:06:23.363 :: Sending project content to craft:ubuntu-24.04...
2026-09-28 17:06:23.742 :: Discarding craft:ubuntu-24.04, cannot send project content: remote directory /root/proj on craft:ubuntu-24.04-64 is not empty
2026-09-28 17:06:26.781 :: Cannot allocate craft:ubuntu-24.04 after too many retries.
2026-09-28 17:06:26.781 :: Successful tasks: 0
2026-09-28 17:06:26.781 :: Aborted tasks: 1
2026-09-28 17:06:26.781 :: error: unsuccessful run