### Bug Description If the `spread/.extension` file in a project is not marked as executable, the `test` command fails with an unhelpful message. ### To Reproduce 1. Clone a repository with working tests 2. `chmod -x spread/.extension` 3. Run the tests ### part yaml ```shell ``` ### Relevant log output ```shell 2025-07-29 20:49:59.917 Processing spread.yaml. 2025-07-29 20:49:59.920 Running spread tests. 2025-07-29 20:49:59.920 Running spread as: /snap/charmcraft/x6/libexec/charmcraft/craft.spread craft 2025-07-29 20:49:59.920 Running spread tests 2025-07-29 20:49:59.970 :: 2025-07-29 20:49:59 Project content is packed for delivery (1.15MB). 2025-07-29 20:49:59.971 :: 2025-07-29 20:49:59 If killed, discard servers with: spread -reuse-pid=64171 -discard 2025-07-29 20:49:59.971 :: 2025-07-29 20:49:59 Allocating craft:ubuntu-24.04... 2025-07-29 20:49:59.976 :: 2025-07-29 20:49:59 Cannot allocate craft:ubuntu-24.04: backend "craft" allocate must print ADDRESS=<SSH address> to stdout, got: "" 2025-07-29 20:50:14.975 :: 2025-07-29 20:50:14 Cannot allocate craft:ubuntu-24.04: backend "craft" allocate must print ADDRESS=<SSH address> to stdout, got: "" 2025-07-29 20:50:29.971 :: 2025-07-29 20:50:29 Cannot allocate craft:ubuntu-24.04: backend "craft" allocate must print ADDRESS=<SSH address> to stdout, got: "" ^C2025-07-29 20:50:37.599 :: 2025-07-29 20:50:37 Successful tasks: 0 2025-07-29 20:50:37.599 :: 2025-07-29 20:50:37 Aborted tasks: 1 2025-07-29 20:50:37.599 :: error: unsuccessful run ```