api: ensure parent of socket dir exists

When creating the socket directory, i.e. in the case that it was
not specified directly, ensure the parent directories exist.
Make it possible to override that parent directory.

osbuild/api.py

@@ -64,9 +64,10 @@ class BaseAPI(abc.ABC):
         """Called after the event loop is shut down"""
 
     @classmethod
-    def _make_socket_dir(cls):
+    def _make_socket_dir(cls, rundir: PathLike = "/run/osbuild"):
         """Called to create the temporary socket dir"""
-        return tempfile.TemporaryDirectory(prefix="api-", dir="/run/osbuild")
+        os.makedirs(rundir, exist_ok=True)
+        return tempfile.TemporaryDirectory(prefix="api-", dir=rundir)
 
     def _dispatch(self, sock: jsoncomm.Socket):
         """Called when data is available on the socket"""