specs: move legacy forward-char tests to specl.

* lib/zmacs/specs/zlisp_move_forward_spec.yaml: Specify behaviour
of zlisp forward-char function.
* tests/forward-char.el, tests/forward-char.output: Remove.
* tests/interactive/forward-char.el,
tests/interactive/forward-char.output: Remove.
* lib/zmacs/zlisp/move.zl (forward-char): Display correct move
past boundary error message.