Test suite (make test) could be improved and referenced in docs

[eduble]

When a test cannot be run (no switch to run the test on, expect or nc software missing, other cases?), the test should appear as "skipped" and not "failed". The reason should be printed, but not the whole test output.
It should be possible to run the test suite without being root. "psql walt" command could be replaced by "walt advanced sql" for this to work.
When installing the server in dev mode (install from fresh debian), running "make test" could be proposed in the doc.