test/synth: move to `slidetool test deps`

Provide a more accessible way to run self-tests in an installed OpenSlide,
rather than `OPENSLIDE_DEBUG=synthetic slidetool prop list ""`.  Omit it
from the man page for now, and hide the `test` subcommand from the
top-level subcommand list by omitting the summary string.

Have `meson test` run self-tests via the new subcommand.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>