ci: fix Windows tests silently passing on failure (#2294)
PowerShell doesn't stop on non-zero exit codes by default. `go test`
failures were ignored, and the step reported success because the
subsequent caddy tests passed.
Caught on this job, which fails but is green:
https://github.com/php/frankenphp/actions/runs/23424633971/job/68136742625?pr=2287
