IDLE: Simplify DynOptionsMenu __init__code (GH-101371)

Refactor DynOptionMenu's initializer to not copy  kwargs dict and use subscripting;
improve its htest.

(cherry picked from commit c41af812c948ec3cb07b2ef7a46a238f5cab3dc2)

Co-authored-by: JosephSBoyle <48555120+JosephSBoyle@users.noreply.github.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>