Abort clean when a plugin spec fails to parse

An unparseable entry dropped out of the wanted set, so clean deleted the
directory and lock entry of a plugin the user still has configured.

Add tests/init_spec.lua covering setup, sync, clean and list.

lua/arti-pack/init.lua

@@ -119,7 +119,16 @@ function M.sync(opts)
 end
 
 function M.clean()
-  local specs = spec_mod.parse_all(M.config.plugins)
+  local specs, errors = spec_mod.parse_all(M.config.plugins)
+  for _, err in ipairs(errors) do
+    notify(err, vim.log.levels.ERROR)
+  end
+  -- An unparseable entry still names a plugin the user wants, so the set of
+  -- wanted names is incomplete; deleting from it would remove a live install.
+  if #errors > 0 then
+    return notify('clean aborted, no plugin spec may be in error', vim.log.levels.WARN)
+  end
+
   local wanted = {}
   for _, spec in ipairs(specs) do
     wanted[spec.name] = true