[backends/vala] Use vapi file  for config

This fixes a distcheck issue where due to a newly generated config.vala, all
vala files were going to be rebuild (instead of using pre-generated c files)
[backends/vala] Depend on vapis in SOURCES instead of stamp rules
[backends/go] Add deps to EXTRA_DIST
[tests] Distribute test files using EXTRA_DIST
[tests] Run service test using full $(srcdir) path