diagnostics: Add --diagnose command line option
diagnostics: Add --diagnose command line option

- When this option is provided, Plinth will initialize, load modules,
  execute diagnostics, print the results on console and quit.

- Replacement for /usr/lib/freedombox/testsuite

jvalleroy/plinth-debian,freedomboxtwh/Plinth,freedomboxtwh/Plinth,vignanl/Plinth,jvalleroy/plinth-debian,kkampardi/Plinth,harry-7/Plinth,harry-7/Plinth,harry-7/Plinth,jvalleroy/plinth-debian,kkampardi/Plinth,harry-7/Plinth,freedomboxtwh/Plinth,kkampardi/Plinth,jvalleroy/plinth-debian,freedomboxtwh/Plinth,harry-7/Plinth,jvalleroy/plinth-debian,vignanl/Plinth,vignanl/Plinth,freedomboxtwh/Plinth,vignanl/Plinth,vignanl/Plinth,kkampardi/Plinth,kkampardi/Plinth