fix(test): fail clearly when the VM forward port is taken

A test run launched qemu without first checking the SSH forward port, so a collision with another VM already holding it surfaced only as an opaque "Failed to start VM," with qemu unable to bind and no hint why. I added a port_in_use check in run_test before the launch: it errors with the port number and the SSH_PORT override to set, records the failure, and moves on.

The check lives in run_test, not start_vm, because start_vm runs in a command substitution (vm_pid=$(start_vm ...)) where this harness's non-exiting error() would be captured as the PID instead of failing the run. The pure half, port_listening_in, takes an `ss -tln` snapshot as a string so it's unit-testable.

0 files changed, 0 insertions, 0 deletions