Root cause: synth-diagnostic uses systemctl start to start pipeline
services, but those services had After=synth-diagnostic — causing
deadlock (they waited for synth-diagnostic to exit, which was waiting
for them).
Fix: pipeline services no longer list synth-diagnostic in After=.
synth-diagnostic has Before= them for boot ordering, and starts them
directly via systemctl start in run-tests.sh.
Also fixed prometheus-exporter: had After=synth-diagnostic backwards
— should start before synth-diagnostic (synth-diagnostic has
After=prometheus-exporter).
david